Book - The Pineal Organ (1940) 12
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The Pineal Organ - Eyes of Invertebrates
Chapter 12 The Eyes of Molluscs
Having considered the geological evidence of the existence of median and lateral eyes afforded by the extinct Order of the Trilobites, which may be regarded as the precursors of certain living representatives of the Arachnida and Crustacea and which also have affinities with the Eurypteridae, an extinct Order of the Arachnida and the Xiphosura, we will turn our attention to the Mollusca, in which phylum we meet with a great variety of types of eyes, from the simple epithelial pit (limpet) or simple camera type of eye without lens, as in Nautilus, to highly differentiated organs such as are found in the cuttlefish, squids, and octopus. In many of the Mollusca there is a hard shell which can be preserved in a fossil state ; and since many of the fossil shells resemble those of living species, they give a clear indication of the type of animal which was enclosed within the shell, notwithstanding the fact that many of these fossil shells have been found in Palaeozoic strata as far back as the Cambrian period. Moreover, it is significant that living species possessing simpler types of eye, such as the limpet (Patella) and the pearly Nautilus, are represented by fossil shells found in the older Palaeozoic strata, while the more highly evolved types of Mollusca, such as Sepia and the octopuses, having complex eyes, are represented by fossil relics which are found only in the more recent periods.
In the Class Pelecypoda (Lamellibranchiata), which includes the " right and left " bivalved shell-fish such as the mussels, cockles, oysters, and scallops, the sense organs are not highly developed. This is due to the fixed or non-motile condition of the adult animal, which usually lies on one side at the bottom of the water. In some, e.g. Pecten (Fig. 1 06), marginal eyes of a highly differentiated type are found round the edge of the mantle (Fig. 107). Nevertheless, whether eyes are present in the adult animal or not, their free-swimming larvae pass through trochophore and veliger stages in which an apical plate, cerebral ganglia, cerebral pit, and statocyst are developed (Figs. 108 and 109). These resemble in their essential characters the veliger larvae of other Classes of Mollusca, such as the Amphineura, which includes the Chitons — " coatof-mail shells " — and the Class Gasteropoda, comprising the whelks, snails, and limpets, in which paired eyes of a simple upright type are found both in the adult and larval form of the animal (Figs, no, ill).
The existence of the same types of shell — or parts of the shell, such as the operculum — in the living and fossil representatives of a species indicates a persistence of type which has gone on through countless ages,
Fig. 106. — Drawing of a Scallop (Pecten), showing the Position of Pallial Eyes. (After Pelseneer.) An. : anal aperture. Ext. r. ap. : external renal aperture. Ft. : foot.
G.$, G.$ / male and female parts of gonad. Ht. : heart. Li. : liver. P. : palp.
Pal. eyes : pallial eyes. R. Ft. : right foot. Sm. Ad. : smooth adductor muscle. Str. Ad. : striated adductor muscle. St. : stomach. V.g. : visceral ganglion.
and also allows of the inference being made that the primary senseorgans, such as the paired ocelli and statocysts of the veliger larvae of the living animals, were present also in the larva; of the corresponding fossil representatives of these species. Further, it is from these simpler types of eye which are common to the trochophore and veliger larva; of widely divergent Classes of Invertebrates and the Tornaria of the Protochordata that we must look for the parent stem possessing a simple form of eye from which the more highly differentiated types of simple ocelli, compound, aggregate, and inverted eyes have been evolved.
Fig. 107. — Section through one of the Eyes on the Outer Margin of the Pallium of a Scallop (Pecten). (After Patten.)
The optic nerve divides into two branches, one of these passes to the inner, or posterior, pole of the eyeball, where it divides into fibres which spread out on the inner surface of the optic plate, and radiating in all directions pass to the peripheral margin of the plate. Here they turn round its edge and form a nerve-fibre layer containing ganglion cells, in front of the retinal cells, which they enter on their superficial surface. The other branch passes directly to the edge of the plate, having reached the margin it bends round this and breaks up into branches which form a superficial nerve-fibre layer containing ganglion cells in front of the retinal cells.
bl. s. : blood sinus. c. : cornea. ep. : epithelium. g.c. : ganglion cells. /. : lens of cellular type. n. 1 , n.- : branches and nerve fibres of optic nerve.
op. n. : optic nerve.
pig. I. : pigment layer.
ret. : retinulas.
rh. : rhabdomes.
tap. : tapetum.
Fig. 108. Frontal section through the pretrochal region of an old veliger larva of Patella ccerulea, showing what are regarded as the rudiments of the cerebral ganglia, eg., at the sides of the apical plate, ap. V. : velum. (After Patten, from Textbook of Embroyology, Vol. 1 : Invertebrata — E. W.
Fig. 109. — Veliger Stage of Vermetus, a Worm-like Gasteropod which in the Adult Animal is enclosed in a Tubular Shell. (After LacazeDuthiers.) In the veliger the shell is spiral and there are paired eyes and statocysts, the latter being situated in the typical position behind the former. cer. g. : cerebral ganglion. sh. : spiral shell.
eye : ocellus. st. c. : statocyst.
ft. : foot. vel. : velum.
mo. : mouth.
When differentiation in any particular line has already taken place and become established, it is probable that — provided the environment remains the same — it will persist, and that the development for instance, of faceted eyes in one class will permanently distinguish that class from another in which inverted eyes have been evolved. The branch of the phylogenetic tree or class of animal which has developed faceted eyes cannot be the parent stock of the branch which has evolved inverted eyes, although both branches spring from a common trunk which includes animals having a simple form of light-perceiving organ, which is capable
Fig. 1 10. — Sketch of a Whelk (Buccinum undatum) in motion.
eye at base of tentacle. /. : foot. op. : operculum, p. : proboscis. s. : respiratory siphon or tube by which water is admitted to the gills. (After Nicholson.)
Fig. hi. — Helix nemoralis, Snail, showing Paired, Simple Eyes at the Free Ends of the Tentacles. (From Parker and Has well.)
an. : anus ; gen. ap. : genital aperture ; oc. tent. : ocular tentacle ; pulm. sac : pulmonary sac ; tent. : tentacle.
of evolving in either direction, or may persist in its original form, as seems to have been the case in the limpets, in which the two bilateral cephalic eyes of the fully developed living types have the form of a simple open pit, without lens.
It is noteworthy that the nautiloid group, which was abundant in the Palaeozoic era, included during this period a great variety of forms — the shell being straight in Orthoceras — curved in Phragmoceras ; in the form of a flat spiral with the turns not in contact ; or in a close spiral, as in Nautilus ; or in the form of a cork-screw helix. Nautilus is apparently the only representative of the order living at the present day, although over 1,000 different species of Nautilidae and Orthoceratidse were found by Barrande in the Silurian basin of Bohemia alone. The Ammonites, which are closely allied to the Nautilidae, were also very varied in type. The condition of the eyes in the extinct Nautilidae and Ammonites is not certainly known ; it is probable, however, that they were of the same simple type (see Chap. 3, p. 48, and Figs. 112, 113) as in many of the simpler univalve molluscs and their living representative Nautilus.
Fig. 112. — Section of Eye of Nautilus, showing i : Cavity of Optic Pit; 2 : Layer of Rods ; 3 : Pigment Layer ; 4 : Layer of Sensory Cells ; 5 : Layer of Ganglion Cells ; 6 : Branches of the Optic Nerve ; 7 : Epidermis ; 8 : Nerves of Cuticle ; 9 : Opening by which the Optic Cavity communicates with the Surrounding Sea- water. (After Hensen.)
c. ap. : central aperture.
cc. : central- cavity.
ep. : epithelium.
f.o.n. : fibres of optic nerve.
g. c. : ganglion cells.
nf.c. : nerve-fibres of cuticular epithelium.
p. : pigment.
re. : sensory retinal cells.
rh. : rods.
Moreover, the habits and the conditions of life of these animals, who lay confined within a rigid shell in which they were carried about by currents in the water, irrespective of any active or purposeful movements on their own part, contrasts markedly with that of the rapid and active movements of the carnivorous octopuses and squids in which highly differentiated eyes have been evolved, and it seems probable that the restriction of movement, combined with imperfect vision and almost complete dependence for food on external circumstances, may have had a very considerable influence in bringing about the gradual extinction of these two Orders. The Nautilidae and Ammonites are a side-branch of the class Mollusca which has almost completely disappeared, but the preservation of the simple type of eye found in Nautilus, which has neither lens nor any power of independent movement of the eyeball, well illustrates the way in which an organ which appears to be of subsidiary importance to the life of
Fig. 113. Drawing taken from photograph of a living specimen of Nautilus pompihus, showing the position of the eye when the animal is in the water. (After A. Willey, Q. J. Micro. Sc. 39.) Note. — The groove which runs vertically downwards from the central aperture to the lower margin of the eye.
the animal as a whole, may be retained as an hereditary structure through millions of years, without — it may be presumed — having undergone any marked change either in the way of evolution or devolution.
A similar preservation of a simple type of eye, adapted to simple needs, has presumably occurred in other types of shellfish belonging to the Class Mollusca and throughout the Invertebrate Kingdom, in all the simpler and less specialized types of animals in which eyes have been evolved ; and it may be inferred from the similarity of the shells of extinct animals and those of the present day that the adult and larval forms of the extinct species correspond with the adult and larval forms of the living, not only in general but also in detail — for example, the structure and relative position of the ocelli and statocysts of the veliger larvae of the extinct parent stock with those of the living species ; moreover, when we compare the trochophore and veliger larvae of the Mollusca with those of other classes of invertebrate animals, one cannot fail to be impressed with the essential uniformity of type, in the general appearance and structure of these larvae, notwithstanding the wide differences which exist in the adult animals. In illustration of this we need only refer to the trochophore larvae of the Brachiopods belonging to the phylum Molluscoidea, those of certain of the Annulata and of the Rotifers (see pp. 88, 91, 92, Figs. 56, Chap. 7 ; 58, 59, Chap. 8). In contrast with the similarity in form of the larvae of these very different types of animal, it is especially noticeable how changes in the habits and conditions of the adult animal are associated with adaptive changes in special organs, e.g. the development of the highly complicated faceted eyes of some insects, which replace the simple eyes of the larva, e.g. in Dytiscus marginalis (Figs. 78, 79 and 80, Chap. 11, pp. 117, 118). The simple eyes of the larva which were present before metamorphosis had taken place while the larva was living in the water, being followed by an aggregate or composite type of eye, adapted to the special needs of the fully developed animal, which is capable of living on land or flying quickly in the air. These changes occur specially in the higher and more recently evolved types of animal, as compared with the more simple and ancient types of shell-fish which we have been considering previously. Moreover, as we have pointed out in a previous chapter (p. 26), along with the specialization of the lateral eyes of these more recent types there has in some cases been a regression of the paired median eyes, accompanied by fusion of these in the median plane and presumably also diminution or loss of function. Moreover, as we shall see later, palaeontological evidence indicates that similar regressive changes have taken place in the paired median eyes of vertebrates.
The phylum Mollusca includes bivalved shellfish such as the mussels, cockles, and oysters belonging to the class Lamellibranchiata (Pelycypoda) ; the Chitons or coat-of-mail shellfish (Amphineura) ; animals with spiral univalve shells, including whelks, snails, and slugs (Gastropoda) ; the elephant's-tusk shells (Scaphopoda) ; and the cuttlefishes, squids, octopods, and Nautili (Cephalopoda).
The eyes, as we have already mentioned, vary markedly both in position and in structure. Thus, they may appear in the usual place on either side of the fore part of the head ; on the back, as in Chiton ; or around the edge of the mantle, as in the scallop or in the thorny-oyster. They may be of the simple, upright type or inverted. The lens may be cuticular or cellular in type. The simple eyes may be open pits without a lens or the pit may be constricted off from the surface and form a vesicle covered by a " secondary cornea " and enclosing a vitreous or a cellular lens. The eyes may be absent in the adult animal, but a pair of ocelli may be present in the trochophore larva or in the veliger stage of development. In the Cephalopods the eyes may be highly complex, as in Sepia, having a retina of the upright type, a vitreous chamber, a biconvex double lens, " ciliary body," " iris," anterior chamber, cornea, sclerotic cartilages, ocular muscles, and palpebral folds, or a simple unclosed vesicular pit, open to the surface and thus containing sea-water as in Nautilus. An inverted type of retina is, moreover, present in Onchidium (Fig. 39, Chap. 3, p. 54), an aberrant type of snail in which eyes are present on the back of the animal and the nerve fibres lie in front of the retinal cells and rods ; the lens is formed of two (or more) enormous clear cells which fill the whole of the interior of the eye, occupying the space which in a vertebrate eye would be filled by the vitreous humour and lens. The resemblance to a vertebrate eye is further simulated by the way in which the fibres of the optic nerve appear to perforate the retina at the posterior pole of the eyeball.
The Eyes of Bivalved Shellfish (Pelycypoda)
In the freshwater mussels (Anodonta or Unio) and cockles (Cardium) no eyes are present in the adult animal and none have been described in the larval stage, although a typical trochophore larva is developed with an apical plate bearing a vertical tuft of hairs beneath which is a cerebral ganglion, while surrounding the plate is a prototroch or girdle of ciliated cells, as in Patella (Fig. 114).
In the scallop, or Pecten, Fig. 106, eyes are present round the edge of the mantle. These are peculiar in having an inverted retina and a cellular lens . They probably originate as a special modification of certain tentacles . Each eye forms a dome-shaped projection about the size of a pin's-head. Enclosed within this is an almost spherical vesicle (Fig. 107) the inner half of which is formed by the retina, while the outer includes the cornea and lens. The retina is slightly concave in front where it comes in contact with the lens and is in relation with a circular " blood sinus." It is covered by a basement membrane beneath which is a nerve-fibre layer. The nerve-fibres on one side converge towards a point on the edge of the cup where, joining together, they form one branch of the optic nerve. Other fibres diverge as they pass towards the edge of the cup. On reaching the outer margin of the cup they turn round this, and passing backward on the superficial surface of the sclera converge towards the " posterior " or inner pole of the eye, where they unite to form a second branch of the optic nerve. The two branches join a short distance behind the posterior pole to form a single main optic nerve. Beneath the layer of nerve-fibres is a stratum of large nerve cells which appear to give origin to the optic nervefibres, these fibres springing from the superficial end of the cell, namely that nearest the lens. Deep to these large cells is a plexiform layer containing small nuclei. This is succeeded by a layer of refractile rods which are separated by a cleft from a tapetum and a pigment layer. The whole is enclosed by a fibrous sheath surrounded by a loose mesenchyme containing blood-vessels. The cellular lens is highly convex on its deep surface, slightly convex superficially, where it is covered by a transparent
Fig. 114. — Trochophore Larva of Patella coerulea.
Ventral side showing : ap., apical plate ; blp., blastopore ; /., rudiment of foot ; set., setse. Compare with Figs. 58, 108. (After Patten, redrawn from MacBride.)
cornea. This consists of a thin cuticle covering a layer of hypoderm cells beneath which is a fibrous stratum continuous with and forming part of the fibrous sheath enclosing the eyeball. The clear hypoderm cells of the cornea are continuous circumferentially with the surrounding hypoderm cells which are deeply pigmented and thus function as a fixed iris.
The only cephalic eyes which occur in the class Lamellibranchiata are a pair of ocelli which are found in the bases of the most anterior filament of the inner gill lamina in Myttlus, the common sea- water mussel, and some allied genera.
The Eyes of Amphineura
With the exception of the Chitons, or " coat-of-mail shells " (Fig. 115), the Amphineura are a lowly organized class of mollusc in which in the adult animal the region of the head is hardly distinguishable from the body, and it is neither provided with eyes nor tentacles.
The Chitons are characterized by a series of eight overlapping valves, situated one behind another on the back of the animal. These give an appearance of segmentation. This is, however, misleading, as the body itself is not truly segmented. Each plate or valve is developed as a separate scale, the scales later uniting to form a jointed shield or lorica.
Fig. 115. Dorsal aspect of Chiton spinosus, a mollusc in which the head of the adult animal has no eyes, tentacles, or statocysts. Sensory organs are, however, found in canals which are present in the superficial layer of the shell valves which cover the back. Some of the larger of these organs have the structure of an eye, having a cornea, lens, iris, pigment layer, and retina.
(From Cambridge Natural History.)
From beneath the edges of the lorica bristles project, which, diverging from one another in all directions, give a brush-like appearance to the animal. The superficial cuticular layer of the scales is perforated by small vertical canals which lodge sense-organs. These are called " micraesthetes" and " megalaesthetes," and some of the latter are especially differentiated as eyes, having a cornea, lens, sensory retinal cells, a pigment layer, and iris.
Ocelli are also present in the trochophore larva (Fig. 116).
The arrangement and structure of the tubular system and sensory organs of Chiton somewhat resemble the sensory organs of the lateral line system of vertebrates, but they differ in certain respects and are considered to have been evolved independently and not to be homologous.
Fig. 116. — Trochophore Larva of a Chiton, showing Apical Tuft of Hairs and Pair of Ocelli. (After Kowalewsky.)
The Eyes of Gasteropods
The gasteropods are univalve shellfish and they include such wellknown types as the periwinkles, whelks, snails, and slugs. In most cases the eyes, which are of the simple upright type, are situated on tubercles at the bases of the tentacles or appear as slight projections near the middle of the tentacle (Fig. no), but in the snails and slugs the eyes are borne on the ends of a second pair of tentacles, which are longer than and are placed behind the first pair (Fig. in). The eyes of all are developed from pit-like depressions of the epidermis, and since the developmental stages of the more complex types are indicated by the adult form of the lower and simpler types, we shall consider the latter first.
1. Patella, the Common Limpet (Fig. 33, Chap. 3, p. 48). — Each eye consists of a pit-like depression of the cuticle. The cells lining the pit are elongated and continuous externally with nerve-fibres, which join to form the optic nerve. The inner ends of the cells which are directed towards the hollow of the pit are clear and refractile ; the central part of each cell is pigmented, while the outer part which contains the nucleus is clear.
2. Trochus. — The spiral shell of this mollusc is conical in form and since when the outer covering is removed a bright pearly surface is exposed, it is much used for decorative purposes. As in the former type the eyes are formed by a depression of the cuticular epithelium. They differ, however, in that the mouth of the pit has become constricted by the ingrowth of a circular fold of the cuticular epithelium, which reduces the size of the opening to a small pupillary aperture (Fig. 34, Chap. 3, p. 48). Moreover, the interior of the vesicle is completely filled with a clear, gelatinous material which is apparently secreted by the cells which line the vesicle. This is described as the vitreous humour.
3. Murex. — The ornamental spiral shells of this genus have long tapering spines and are commonly known as " venus's combs." In this type the eyes are more highly organized than in the former examples, the mouth of the pit has become closed and the pit is constricted off from the cuticle so as to form a vesicle (Fig. 35, Chap. 3, p. 49). The
Fig. 117. — A: Veliger Larva of Patella ccerulea viewed from the Left Side showing Ocelli on Apical Plate. B : Dorsal View of Acmaa virginea, just after metamorphosis, in which the ocelli are seen to be covered by the margin of the shell. the larval shell is retained as an Apical Knob, which will be cast off later. (Redrawn from MacBride : A, after Patten ; B, after Boutan.) ft. : foot. oc. : ocellus.
gl. : glands in roof of mantle. p.g. : rudiment of pedal ganglion.
/. sh. : larval shell. tent. : tentacle.
me. : mantle cavity. vel.
velum, visceral hump.
outer lamina of the circular cuticular fold joins over the superficial aspect of the vesicle and forms with a layer of ingrowing mesenchyme what is termed the " secondary cornea," whereas the outer part of the epithelial wall of the optic vesicle, which remains thin and transparent, is called the " primary cornea." The remaining inner and larger part of the epithelial wall of the vesicle is differentiated into a retina of the same type as in Trochus and Patella, consisting of well-defined bacillary, pigment, and nuclear layers. Within the cavity of the optic vesicle the contents have become differentiated into a more solid refractile body, the lens, and a more fluid part which surrounds the lens and, like the vitreous of the vertebrate eye, intervenes between the lens and the retina. Both lens and vitreous are formed by modification of the secretion of the cells lining the vesicle, and a lens of this type is known as a vitreous or non-cellular lens. The development of gasteropods has been worked out by Patten, Boutan, and others, more especially in Patella and Acmcea, a nearly allied species. A typical trochophore larva is formed with an apical plate bearing a vertical tuft of cilia, beneath which there is developed from the ectoderm a cerebral ganglion (Fig. 114, p. 156). The ocelli are developed from the same ectodermal layer, just behind and lateral to the bases of the tentacular processes, and are well seen in the veliger larva of Acmcea virginea (Fig. 118, A). At a later stage the velum or prototroch is shed and the eyes are covered by the anterior margin of the shell (Fig. 118, B). The relation of the ocelli to tentacles, velum, mouth, statocysts, and foot are well shown in Fig. 109, p. 150), showing the veliger stage of development in Ver metes. This is a worm-like gasteropod characterized by having a spiral shell in the larva which is replaced in the adult by a straight, tubular shell.
Fig. 118. — Advanced Stages of Development of Acjvuea virginea. A: Lateral View of Veliger Larva. B : Dorsal View of Young Acmjea,
AFTER THE VELUM HAS BEEN CAST
an. : anus.
ft. : foot.
gl. : glands in roof of mantle.
int. : intestine.
oc. : ocellus.
op. : operculum.
After the velum has been cast off the young mollusc sinks to the bottom ; the periostome enlarges and gives rise to the conical adult shell, the margin of which overlaps the eyes.
(After Boutan — redrawn from MacBride.)
m. : retractor muscle.
Fossil shells of gasteropods are found in the earliest Palaeozoic rocks, and it is probable that the animals contained within them differed little from those living at the present time, and that tentacles, ocelli, and statocysts were developed in them in the same relative positions as in living animals of the same type.
These are worm-like molluscs and include the elephant-tusk shells, or Dentalium. The animal is enclosed in a slightly curved tubular shell which resembles in form an elephant's tusk or tooth. No eyes are found in the adult animal, but statocysts are present in the larva of Dentalium, which passes through typical trochophore and veliger stages.
The Eyes of Cephalopods
These vary from simple optic pits containing sea-water such as are present in Nautilus, to the highly organized eyes of the cuttlefish, squids, and octopuses.
We shall commence our description with the simplest form, namely that of Nautilus. The eyes are of large size and paired. When the animal is swimming in the water, they appear in a triangular space on each side between the hood above, the bases of the tentacles in front, and the edges of the mantle and opening of the shell below (Fig. 113, p. 153). Each eye has a central opening which leads directly into the cavity of the optic vesicle, which is thus filled with sea-water. From this a shallow groove runs downward to a rim-like fold which surrounds the lower part of the eye. The position of this groove suggests the foetal or choroidal fissure of a vertebrate eye. There has, however, been no inversion of the outer wall of an optic vesicle, such as occurs in the eyes of vertebrates, and the wall of the optic vesicle in Nautilus consists of a single layer, whereas the secondary optic vesicle of the lateral eyes of vertebrates is bilaminar. Although the development of Nautilus has not, so far as we are aware, yet been described, it may be assumed that the eye is developed by inversion of the superficial layer of the epidermis, so as to form a simple pit as it does in the initial stage of development in other molluscs.
The microscopic structure of the adult eye of Nautilus has been described by Henle. The wall of the cup consists from within outwards (Fig. 112, p. 152) of a bacillary layer formed by the inner ends of the retinal cells, a pigment layer, outside which is a single layer of large nuclei contained in the basal part of the retinal cells, this is succeeded by an 11
Fig. 119 Nautilus pompilius, showing the Central Apertures of the Eyes. (After Willey, from Parker and Haswell's Textbook of Zoology.)
The flattened external surface of the eye is bounded by a slightly raised rim, which extends round the posterior half of its margin. From this a narrow groove extends inwards to the central opening, which has some resemblance to the choroidal fissure of the lateral vertebrate eye. The eye of Nautilus is, however, extremely simple, consisting of an open cup, the cavity of which is filled with sea-water. It has neither lens, iris, nor vitreous humour. The rods or inner refractile segments of the cells are directed towards the light and the nerve fibres originate from the opposite end. The interior of the mantle cavity is exposed, the postero-ventral wall having been reflected.
oral left renal aperture.
a. I. neph.
an. : anus.
ant. os. : right oral osphradium (olfactory organ).
cten. : ctenidia or branchial organs.
ey. : eye.
f. : funnel.
gr. : groove leading to central aperture of eye.
/. 6 . ap. : bristle passed into left reproductive aperture.
/. vise. ap. : left viscero-pericardial aperture.
pen. : penis.
pt. neph. : aboral left renal aperture.
p. os. : aboral osphradia.
sp. s. : spermatophoral sac.
v.n. : visceral nerves.
outermost layer of small scattered nuclei and nerve-fibres ; the latter pierce an external limiting membrane and pass inwards through the optic stalk to the cerebral ganglion (anterior part of the oesophageal ring). In the interior of the optic cup there is neither vitreous, lens, nor iris, and as an optical instrument it is comparable to a camera of the simple pill-box type. Nautilus (Fig. 119) with its simple type of eyes is especially interesting from the phylogenetic standpoint ; it is the only living representative of the extinct Nautiloid tetrabranchiata, which were abundant in the Palaeozoic epoch ; the earliest representatives being found in the Cambrian rocks. It also closely resembles the Ammonites, but there are certain differences in the shell, and it is doubtful whether the Ammonites were tetrabranchiate, like Nautilus, or dibranchiate. This difference, however, is not so fundamental as might be expected from the differences in structure which exist between the branchiae and eyes of living representatives of these fossils, since there are many points of agreement between dibranchiate and tetrabranchiate cephalopods which outweigh such differences as have occurred in the long geological period which has elapsed since the fossil representatives of the living genera were alive. Incidentally one may mention, as an example of an important structural similarity between these two widely separated orders, the occurrence of a cartilaginous endoskeleton — the " cranial cartilage," Fig. 120 — which is present in both Nautilus and Sepia. It is noteworthy that the cranial cartilages of Nautilus and Sepia resemble in certain respects the endoskeleton of some chaetopods, Crustacea, and arachnids, e.g. the entosternite of Scorpio and Limulus.
Fig. 120. — Cartilaginous Internal Skeleton of Nautilus pompilius.
On the dorsal aspect of the cranial cartilage of Sepia, overlying and protecting the cerebral ganglia is another endoskeletal structure, namely the nuchal cartilage. The hypothetical significance of the existence of these skeletal elements in invertebrates will be discussed later in connection with Gaskell's comparison of the entosternite of Limulus with the cartilaginous skeleton of Ammocoetes.
The Eyes of Sepia
The conspicuous large eyes of the cuttlefish have attracted the attention of many zoologists, among the more recent of whom may be mentioned Faussek (1900) and Koeppern (1909). They are much more highly organized than those of Nautilus and Triton and have a superficial resemblance to the lateral eyes of vertebrates, since they have a lens with a
Fig. 121. — Transverse Section through the Head of an Octopus (Sepiolia)
A.Ch. : anterior chamber. A.S.L. : anterior segment of lens. C.Ep. : cutaneous epithelium. Cil. B. & S.L. : ciliary body and suspensory ligament. F. : cavity of funnel. Inf. C. : infundibular cartilage. Ir. : iris.
M.C.L. : middle cerebral lobe. N.G. : nerve ganglion. O.C.F. : outer cutaneous fold of eye.
OES. : oesophagus.
Op. L. : optic lobe.
P. : pupil.
P. Ch.& V. : posterior chamber and
vitreous body. P.L. : pigment layer of retina. Pr. Co. : primary cornea. P. S.L. : posterior segment of lens. R+GC. : retinal and ganglion cells. Rh. : rods. Sec. Co. : secondary cornea.
The names which are employed to denote the various parts of the invertebrate eye must not be regarded as implying a morphological correspondence with similarly named parts of vertebrate eyes. (R. J. G.)
suspensory mechanism, ciliary body, iris, anterior- and vitreous-chambers, cornea, and a palpebral fold (Fig. 121). They are, however, of the upright type, and are developed as an ingrowth of the epidermis. It must be realized also that the names which have been applied to the various parts do not in most cases denote structures which are strictly homologous with the structures similarly named in the vertebrate eye.
The retina resembles the vertebrate retina in having three principal layers of cell-elements, namely, an inner consisting of the sensory- or visual-cells ; an intermediate resembling the bipolar cells of vertebrates ; and an outer or ganglionic layer. The retina of the cuttlefish is, however, of the upright type, the rods or receptive ends of the visual cells being directed inward towards the source of light, and the ganglion cells and nerve-fibre layer being peripheral, i.e. nearest the fibrous capsule. The retina of Sepia thus agrees with the vertebrate retina in consisting of three principal layers of sensory cells, thus being " compound " in type ; but it differs in being upright as contrasted with the inverted retina of the vertebrate eye.
The general structure of the retina will be most easily understood by a reference to its early stages of development, as shown in (Fig. 36, A, B, C, D, E, Chap. 3, p. 50). The eye commences as a simple pit — optic cup ; this is lined by a single layer of cubical cells, continuous at the mouth of the pit with the epithelium covering the surface of the body. The mouth of the pit then becomes constricted, and later an optic vesicle is cut off from the exterior. The deeper cells of the optic vesicle, which will give rise to the retina, become columnar and develop thread-like processes which project into the cavity of the vesicle. The cells of the superficial segment of the wall of the vesicle become flattened and form the inner epithelial layer of the future corpus epitheliale. Superficial to the optic vesicle is a mesodermal layer covered externally by the epithelium of the body- wall. The latter forms the outer epithelial layer of the future corpus epitheliale and the two epithelial layers with the mesoderm between them form the primary cornea. Around this a circular fold rises up and grows inwards over the developing lens ; this is known as the iris fold (C, D, E). The epithelial body consists of a central portion which is primarily concerned in the secretion of the posterior segment of the lens but later forms a septum between the two segments of the lens and a peripheral portion which is composed of large clear cells, some of which by secretion and also by degeneration give rise to both segments of the non-cellular lens ; others which are of small size contribute to the later stages of development of the lens. In the retina, according to Faussek, the primarily tall columnar cells of the inner segment of the optic vesicle, which at first form a single layer resting on the inner surface of the basement membrane, proliferate and give rise to many layers of small round cells, the outer of which pass through the basement membrane and form the intermediate and ganglionic layers of the retina. The inner ends of the receptive cells, however, retain their primary position inside the basement membrane and form the layer of visual-rods. Between the visual cells are a certain number of cells which develop pigment. These form a conspicuous band external to the bacillary layer (Fig. 121) ; outside the pigment layer is a narrow, clear zone of nerve-fibres — inner plexiform layer ; this is succeeded by a wide intermediate band of small, round nuclei — middle nuclear layer — another layer of fine nerve-fibres — outer plexiform layer — and finally a layer of ganglion cells, appearing in the greater part of its extent as a single
Fig. 122. — Scheme of the Visual Paths and their Central Connections in a Cephalopod, combined from von Lenhossek, Kopsch, and Cajal. According to Cajal most Bipolar Cells (b) are located in the outer
GRANULAR LAYER. (FROM C.U. ArIENS KAPPERS, SLIGHTLY MODIFIED.)
Axons coming from the retinal cells, decussate in the optic nerve, where they form a chiasma. The inverted image on the retina which results from rays passing through the narrow pupil and lens is thus corrected in the cortex of the optic lobe, a, b, c, d, d 1 , d 2 , d 3 , d*, e, e 1 , e 2 , denote relays of neurons.
stratum of large cells, lying in an outer nerve-fibre layer which is limited by an external basement membrane. The nerve-fibres converge towards, the proximal or posterior pole of the eyeball, where, forming a short, thick optic nerve, they pierce the sclera and enter the optic lobe (Fig. 36, F, Chap. 3, p. 50).
The course of the nerve-fibres, according to Cajal, is indicated in Fig. 122, which shows a decussation of axons coming from the retina in the optic nerve ; this corrects the reversal of the image formed on the retina. In the optic lobe there are two layers of granule cells placed superficially and separated by a plexiform layer of nerve-fibres ; while in the centre is an area consisting of nerve-cells and nerve-fibres, the nerve-cells being chiefly aggregated in a central nucleus and in a peduncular nucleus . From these nuclei relays of cells carry sensory impulses to the middle-anteriorand posterior-cerebral lobes. The terminal fibres of the axons coming from the retina pass into the plexiform layer of fibres between the two granular layers in the cortical part of the optic lobe ; in this layer they communicate with dendrites of cells in both the superficial and deep granular layers ; while the axons of the granule cells convey the impulse to the central and peduncular nuclei.
Development of Cephalopods
The higher Mollusca differ markedly in their development from the lower types, e.g. the univalve shellfishes (Patella, Vermetes, Triton) y Chiton ; Dentalium ; and the bivalve shellfishes such as Pecten. In the higher molluscs there is no trochophore or veliger stage of development. The segmentation is partial : meroblastic — a blastoderm or germinal disc being formed on the surface of a large mass of yolk ; moreover, there are many points in the later stages of development which mark a wide divergence of the higher cephalopods from the more simple representatives of the class.
The trochophore stage is believed to have been completely eliminated during the descent of the higher from the common ancestors of these and of the simpler types ; and also adult features recently acquired by the higher types have been reflected back or impressed on the early stages of development of the higher types. The explanation of some of these changes probably lies, as has been suggested by MacBride, in the effect which " the accession of large stores of nourishment " in the eggs has in " almost obliterating the traces of ancestral history in their development, leaving only the most general resemblance in the formation of the layers and the development of the sense-organs as links between them and other Mollusca."
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