Book - The Pineal Organ (1940) 19
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Chapter 19 The Eyes of Amphibians
This class includes the tailed amphibia — Urodela ; the tailless amphibia — Anura ; the degraded blind and limbless amphibia — Apoda ; and the extinct labyrinthodonts — Stegocephala .
The pineal system of amphibians is particularly interesting on account of the light that it sheds on intermediate stages between the more highly evolved types of the parietal sense organ or pineal eye, such as is found in the lampreys and lizards, and those types in which the end-organ fails to develop and in which it appears that the proximal end only, grows and becomes modified to form the epiphysis or pineal organ as in birds and mammals.
The Pineal System of the Urodela
In this primitive order, which comprises the Tritons ; salamanders ; the common British newts ; Amblystoma (Axolotyl) ; Amphiuma ; Necturus (Fig. 158) ; and Proteus, the pineal organ is very rudimentary and in the adult lies entirely within the cranial cavity. The whole organ consists at first of a flattened hollow vesicle, connected by a short stalk with the roof of the diencephalon, to which it is attached in the usual situation between the habenular and posterior commissures. The superficial or distal wall of the end vesicle is formed by a single layer of epithelium, whereas the deep or proximal wall shows two or three layers of nuclei ; the inner ends of these cells in some specimens, e.g.
Fig. 158. — Median Sagittal Section through the Head of an i8-mm. Larva of Necturus macalatus, showing the Structures in the Parietal Region of the Brain. (After C. S. Minot.)
cp. : posterior commissure. C. PI. : choroid plexus. Ep. : epiphysis. FB. : forebrain. Hyp. : hypophysis.
M. : mouth.
MB. : midbrain.
Op. Ch. : optic chiasma.
P. A. : paraphysal arch.
Par. : paraphysis.
Fig. 159. A — Proteus — Adelsberg grotto, reduced. The lateral eyes are invisible externally. (After Semper.)(After Semper.)
There is a small pear-shaped pineal organ within the cranial cavity. B — Vertical section, through one of the rudimentary lateral eyes.
opt. n. : optic nerve; p.c.+p.i.r. : pars ciliaris+pars iridica retina;. The lips of the optic cup meet in front owing to the absence of the lens. p'.g. : pigment ; v.c. : vitreous cavity. Arrest of development has taken place at an early stage, and the retina is unusually thick.
Salamander maculosa and Proteus anguineus, contain pigment granules ; and in a specimen of Necturus described by Kingsbury (1895), two or three medullated nerve-fibres on each side were seen by him to pass from the brain into the epiphysis. Whether these nerve-fibres were efferent fibres passing from the brain to the epiphysis or afferent fibres passing from a vestigial sensory vesicle to the brain is a problem regarding the solution of which we have no direct evidence which will help us to come to a decision. The presence, however, of nerve-fibres passing between the parietal sense organ and the brain in lampreys and reptiles, and the homologous nerve connecting the vestigial sense organ with the roof of the brain in the common frog do afford indirect evidence which justifies the assumption that these fibres are primarily afferent in nature, although their original function of transmitting impulses from the terminal sense organ to the brain has, like the capacity of the sense organ itself to receive these sensations, ceased long ago.
1. Urodela Caudata
The pineal organ of Urodeles appears in all examples which have hitherto been examined to show signs of degeneration. Thus in Triton, de Graaf found indications of " fatty " degeneration (probably vacuolation of the cells) ; moreover, the cavity of the vesicle was irregularly subdivided by partitions into loculi, the septa being produced by folding of its walls. The vesicle is often dorsoventrally compressed, being flattened out between the cranial wall and roof of the brain, and the lumen is thus reduced to a mere slit. In Desmognathus, according to Fish (1895), no trace of a lumen is to be found in the pineal organ of the adult animal. In Typhlotriton and Proteus anguineus, both of which are cave-dwellers and blind, the lateral eyes are also degenerate and buried beneath the skin of the head. In Proteus (Fig. 159) Galeotti found that the epiphysis was a small, pear-shaped structure showing no evidence of secretory function, but he observed that in certain cells near the nucleus pigment granules were present — this being the only place in which pigment was found to be present in the brain of Proteus. This snake-like amphibian, which lives in the subterranean waters of Carniola and Dalmatia, is in its natural state described as being of " fleshy whiteness and transparency." The absence of pigment in the skin and its presence in the epithelium of the pineal organ appear to indicate a more feebly inherited tendency towards the formation of pigment in the skin and a marked tenacity of the inherited character with respect to the deposit of pigment in the epithelium of the end vesicle of the pineal organ. On the other hand, it may be argued that the pigment in the epithelium of the pineal organ is simply due to chemical changes occurring in a degenerate tissue ; but since the pigment is strictly limited to the epithelium of the pineal organ and is not found in the surrounding mesodermal tissue and other parts of the brain, nor in the body generally, this interpretation does not seem to be the correct explanation of its presence in the pineal organ.
Fig. 160. A median sagittal section of the head of an embryo of Amblystoma punctatum (Axolotl), showing the relations of the paraphysis, par. ; velum, vel. ; and epiphysis, Ep.; Ch. : notochord; c.p. : commissura posterior; hyp. : hypophysis ; mo. : mouth. The dorsal sac will be formed between the velum and epiphysis, and the anterior or habenular commissure in front of the epiphysis. (After J. B. Johnstone.)
A paraphysis (Fig. 160) is usually well developed in the Urodela, and may appear as a simple tubular diverticulum or it may be branched and glandular in appearance.
The parietal foramen which is present in the skull of certain extinct Urodeles has disappeared in living forms, e.g. in the newt (Molge cristata), W. K. Parker, and Typhlototriton, H. Riese. In these, paired frontal and parietal bones are separated by a cruciate suture.
2. Apoda (Caecilia)
The best-known example of this order is the slimy eel-like creature Ichthyophis glutinosa, in which the lateral eyes are very small and are imbedded in the skin. The adult animal has no limbs, but vestiges of the hind limbs have been recognized in the embryo. The pineal organ described in 1891 by Burckhardt is a small, pear-shaped hollow vesicle the walls of which are infolded and the stalk attached to the roof of the diencephalon just behind the habenular commissure. The paraphysis is well developed and viewed in profile has the shape of a geological hammer, the posterior limb of which overlaps and conceals the pineal organ. Some fibres passed between the pineal organ and the roof of the diencephalon. Burckhardt was uncertain, however, whether these were nervous or supporting in nature. The skull described by Sarasin, like that of other living orders of Amphibia, shows no parietal foramen.
In the genera Gymnopis and Herpele the lateral eyes have not only sunk beneath the skin as in some other forms but they actually lie beneath the bones of the skull.
3. Anura (Acaudata)
The development of the pineal organ in tailless amphibia is more pronounced than in Urodela and Apoda, and in many at one period in their development an end vesicle — stalk and proximal part — are distinguishable (Fig. 161). The end vesicle, however, soon becomes cut off from the stalk by the ingrowth of the roof of the skull (Fig. 162), and the stalk disappears leaving only the proximal part or epiphysis in connection with the roof of the diencephalon.
The end vesicle, or parietal organ (Fig. 167, p. 233), is situated beneath the epidermis, being imbedded in the corium and lying completely outside the roof of the skull. Its position in the living animal is indicated by a small, pale area, the parietal spot where the pigment in the sub-epithelial layer of the skin is absent or scanty, and the cutaneous glands are also either absent or are reduced in size and number. The spot is variable in size and degree of distinctness in different species and in some is absent altogether. It is well seen in the common frog, Rana temporaria (Fig. 163), and also in R. subsaltans, of which a drawing clearly showing the " fleck " was published by Gravenhorst in 1829, although its significance was not appreciated at the time ; and later, when Stieda discovered the vesicle beneath it, he considered it to be glandular in nature, and it became known as the " frontal gland " (" Stirndruse "). Goette (187375), who first recognized the existence of the proximal part or epiphysis in the early stages of development of Bombinator igneus, was also the first to establish the connection of this with the end vesicle and homologize these parts with the pineal organ of other classes of vertebrates, although he was in error with regard to the connection which he believed to exist between the pineal organ and the anterior neuropore (see p. 5).
Fig. 161. — The Parietal Region and Pineal Organ of a Tadpole (Rana temporaria) — Internal Gills and Hind Limbs appearing as Stumps. (After Braem, 1898.)
Ch. H. : chiasma habenularis.
Ch. p. : chiasma posterior.
Ds. : dorsal sac.
Ep. : epiphysis.
L. Sn. : lamina supra-neuroporica.
MB. : midbrain.
N. pin. : nervus pinealis or stalk of
pineal organ. Par. : paraphysis. Po. : pineal organ. V. : velum.
Fig. 162. Median longitudinal section of the pineal diverticulum of a frog tadpole, showing a constriction in the plane of the cranial capsule, which indicates the future line of separation of the parietal organ (Stieda) from the epiphysis or pineal sac. The apex of the organ is directed forward, and is to the right in the figure.
(Original R. J. G.)
C. : constriction; Epd. : epidermis; Op. P. St. : opening of pineal stalk ;
St. O. (Pa. O.) : parietal organ (Stieda; ; V. III. : third ventricle.
Fig. 16?. — Head of Frog (Rana temporaria), showing the Parietal Spot which lies over Stieda's Organ and between the Lateral Eyes. (From Studnicka.)
Development. — The pineal diverticulum in Rana is at first elliptical in form, but soon becomes tubular, the expanded blind-end growing forward while the attached end remains at its site of origin, between the habenular and posterior commissures. A constriction then appears marking off the end vesicle from the proximal part, and as the head enlarges and the end vesicle, which is attached to the skin, is carried forward, a narrow neck appears which becomes elongated and forms the stalk. Later the lumen in the stalk disappears and finally the epithelial wall becomes interrupted, and in the fully developed animal the end vesicle or parietal organ is completely separated from the epiphysis by the roof of the skull, which in living representatives of the order has no parietal foramen. The proximal part, or epiphysis, retains its lumen for a considerable period, and this may persist even in the fully grown animal, as in the example shown in Fig. 166, which shows the hollow tubular organ lying on the roof of the diencephalon and extending forward as far as the habenular commissure. The pineal recess lies immediately in front of the posterior commissure and the pineal tract passes backwards on the dorsal aspect of the epiphysis to end in the ventral part of the posterior commissure.
The end vesicle or parietal organ is usually a round, hollow, epithelial cyst, but is sometimes solid. In the former the superficial or distal wall of the cyst is formed of a single layer of epithelium, while the proximal wall shows two or three layers of nuclei, which in Bufo, according to Studnicka, are radially arranged, and may contain pigment granules. The end vesicle of Bufo thus shows a more primitive (less degenerate) condition than in Rana, and resembles somewhat the end vesicle of Petromyzon, but is not so highly differentiated. The cavity of the end vesicle varies considerably in size and shape. In some cases it is irregularly constricted and has the form of the figure eight placed horizontally 00 (R. esculenta) ; or kidney-shaped (Bombinator igneus) (de Graaf) ; or horse-shoe shaped (Leydig). It is sometimes quite irregular in form and crossed by protoplasmic strands. The whole vesicle is enclosed in a loose connective tissue capsule composed of flattened cells, between which and the vesicle is a space which Leydig (1891) has shown in Bombinator is traversed by nerve-fibres and capillary vessels. The superficial surface of the vesicle is formed by a layer of flattened neuro-epithelium (external limiting membrane), which constitutes the inner boundary of the space. As a rule the epithelial cells forming the walls of the end vesicle remain undifferentiated, but in some specimens of adult animals Studnicka has observed at least two layers of cells in the proximal or retinal section and a differentiation of an inner layer of high cylindrical ependymal cells and beneath these (more superficially) rounded cells. Among the former Studnicka describes in Bufo, cells of a distinctly rod-like character, and Galeotti has seen rounded protoplasmic masses projecting into the lumen, like those in Petromyzon. The structure of the vesicle thus resembles a degenerate sense organ rather than a gland, and its cells differ markedly from those of the surrounding cutaneous glands.
Nerves of the Parietal Organ and Epiphysis in Anura
The fine strand of fibres which sometimes connects the " Stirndriise " or " frontal gland " with the roof of the skull was first described by Stieda in 1865. This slender band may contain nerve-fibres, or be composed simply of connective tissue, or it may be absent. It is generally considered to originate as the stalk of the pineal organ which in the course of development has become thinned out and elongated, as represented in Fig. 161 (p. 228) by Braem, of the parietal region of a tadpole of Rana temporaria. Nerve-fibres may appear in the stalk at a later stage and it was thought at the time that the stalk was transformed into the " pineal nerve." Whether the nerve-fibres are actually formed in the stalk or grow into the stalk from neuro-sensory or ganglion cells in the end vesicle is a problem which it is difficult to decide ; for, according to the later publications of Ley dig (1891), the nerve-fibres do not actually enter the vesicle as they appear to do in Fig. 164, but both nerves and vessels pass beneath or around the vesicle and join a plexus of nervefibres and capillary vessels in the surrounding connective tissue (Fig. 165).
Fig. 164. Another drawing of the same specimen as Fig. 165, showing pigment granules in the epithelium of the wall of the frontal organ (Stieda) and the pineal nerve. (After Leydig.) N. pin. : nervus pinealus.
Fig. 165. — The Frontal Organ of Bombinator igneus (Stage of Two-legged
Larva), seen from below.
Surrounding the organ are blood-vessels and nerves. Three-branched pigment cells of the connective tissue type are present, and sections of cutaneous glands. N. pin. : nervus pinealis.
(From a drawing by Leydig, 1891.)
Moreover, neither fully differentiated neuro-sensory cells nor ganglion cells have been described in the end vesicle, but on the contrary in most cases the epithelium shows signs of degeneration and the nerve-fibres or even the whole stalk are frequently found to be absent in adult specimens, a circumstance which suggests that the development of nervefibres when it occurs is merely a transitory phase in the existence of a degenerate organ which is variable in the degree of differentiation to which it attains in different examples of the order or even in individuals of the same species. The occasional presence of nerve-fibres in the stalk of the end vesicle, although not constant, is of morphological importance, and the following detailed description by Braem (1898) of both the extracranial and intracranial course of the pineal nerve is especially interesting. He distinguishes between the " nervus pinealis " of the parietal organ and the " tractus pinealis " of the epiphysis (Fig. 166).
Dealing first with the pineal nerve of Rana, Braem states that it consists of thick medullated fibres accompanied by blood-vessels. These fibres enter the organ from below, either at the middle or near the hinder pole. Having entered the vesicle they gradually become imperceptible and are lost. Connective tissue elements enter slightly into the composition of the nerve, and Studnicka mentions that he has found pigment cells on the surface of the nerve and, further, that he considers the elongated nuclei seen in the course of the nerve to be connective tissue in nature. In any case, there is at least a fine endothelial sheath, which separates Ep. Tr.pin. C.p. MB.
Fig. 166. A — Sagittal section through the Epiphysis (proximal part of the pineal organ)
showing the pineal tract, from an adult example of Rana temporana. B — Transverse section through the epiphysis and roof of the diencephalon.
Ch. H. : Chiasma habenularis. MB. : midbrain. C.p. : commissura posterior. Tr. pin. : tractus pinealis.
Dien. : diencephalon. ///. V. : third ventricle. Ep. : epiphysis.
(After Braem, 1898.)
the nerve and the accompanying blood-vessels from the cranio-dorsal lymph space in which the greater part of the nerve lies. Braem further describes the nerve as passing through the lymph sac and sooner or later piercing the skull in an oblique direction between the two frontal (frontoparietal) bones, close behind the ethmoid and thus just above the olfactory lobe. Having entered the cranial cavity, it courses backwards on the superficial aspect of the dura mater, to which it is closely applied, to the paraphysis. It then passes over the latter to the epiphysis. In its course thither the nerve is pushed for a short distance away from the median line by a dense plexus of blood-vessels. Braem saw the commencement of the nerve near the apex of the epiphysis, but he was unable to see any connection of its fibres with those of the " tractus pinealis," although he considered such a connection was more than probable. Studnicka also mentions that he was unable to trace any connection between the pineal nerve and pineal tract in his specimens of quite young tadpoles of Rana. Braem saw no indications of degeneration ; nevertheless, he considered it possible that the nerve in some cases might be absent, an opinion which is supported by the observations of de Graaf and Leydig, and by a young specimen of a frog tadpole, of our own (Fig. 167).
The course of the pineal tract in an adult specimen of Rana temporaria is shown in Fig. 166 — A, in sagittal section ; and B, in transverse section. It commences on the dorsal aspect of the epiphysis near its apex and courses backwards either on or in the substance of its dorsal wall, and finally enters the ventral part of the posterior commissure. According to Braem the fibres composing the tract arise from ganglion cells in the posterior commissure, while Haller states that the tract arises by two roots from the part of the thalamus which lies medio-ventral to the posterior commissure.
Fig. 167. — Section through Stieda's Organ of a Frog Tadpole, at a Later Stage of Development than that shown in Fig. 164.
The terminal vesicle (parietal organ) is completely cut off from the epiphysis, and careful search through sections behind the vesicle failed to reveal any nerve tract or remnant of the original connection between it and the epiphysis, or brain. (Original, R. J. G.)
Cr. C. : cranial capsule ; Epd. : epidermis ; St. O. : Parietal organ (Stieda).
Gaupp 1 also describes a bundle of nerve-fibres which he saw in one specimen on the ventral side of the stalk of the epiphysis.
The Epiphysis or Proximal Part of the Pineal Organ of Anura
The epiphysis (Fig. 166, p. 232) usually has the form of a hollow tube ending blindly in front and opening posteriorly in the third ventricle, just behind the habenular commissure. Occasionally in the adult animal the lumen has disappeared in its basal part, which thus forms a solid stalk. In transverse section the tube is seen to be oval in outline, as if dorsoventrally compressed, the lower wall being thicker than the upper. On each side a row of short lateral diverticula are present, which according to Braem in Rana are eight in number, the more anterior diverticula being larger than the posterior. The epiphysis thus has a glandular appearance like that in certain teleostean fishes, e.g. Salmo purpuratus. This condition we shall see occurs also in reptiles, birds, and mammals, but in these attains a much higher degree of differentiation.
The Character of the Epithelial Cells forming the Walls of the Epiphysis. — Those next to the lumen tend to become columnar in type, and sometimes bear cilia or cylindrical protoplasmic processes which project into the lumen, as in the stalk of certain teleostean fishes. These have been regarded by Galeotti as secretion processes, but Studnicka states that their rod-like form in the adult animal is more like that of sensoryepithelial cells, such as are seen in the parietal organ of Petromyzon. These cells are especially abundant in the lower wall of the epiphysis, where there is an outer layer of rounded cells in the situation of the ganglion cells of the parietal organ of the lamprey. The cells in the dorsal wall of the epiphysis are, like those of the ventral wall, ependymal in character, but are mostly round. In its distal part the pineal tract is found imbedded in the wall of the tube, but near the base it becomes superficial and lies in a groove on its dorsal aspect. The general impression which is given by examination of the structure of the epiphysis is that of an aborted neuro-epithelial tissue, the attempted differentiation of which has become arrested.
1 E. Gaupp. Zirbel, Parietalorgan und Paraphysis. Ergebnisse der Anat. und Entwickhmgsgeschichte, von Merkel und Bonnet.
The Parietal Foramen of the Labyrinthodonts
The extinct Stegocephala or labyrinthodonts were very variable in size and in shape. Thus, the length of the skull, only, of Mastodonsaurus was 1 -25 metres, while the total length of some of the smaller examples of Melanerpeton is only 25 cm. They were mostly tailed amphibians with two pairs of short limbs, the anterior, the smaller, having four or five toes ; the posterior, the larger, having five toes. Illustrations of the imprints of these feet are familiar to all who are in the least interested in the study of palaeontology. They occur in stratified deposits of fossilized mud, which often shows irregular cracks, and have been found in North America, Thuringia, Saxony, and Bohemia ; and also in the Karoo formation of South Africa. The hand-like form of the footprints of Cheirotherium bar t hi, figured by Owen, are especially remarkable . Another notable characteristic of these amphibia, which has given rise to the name " labyrinthodont," is the remarkable radial arrangement and folding of the dentine which is seen in the larger and geologically later types of Stegocephala. The skull was characterized by a series of dermal plates, firmly fixed to the chondrocranium and grooved by canals of the lateralline system. Some types resembled the more primitive types of fossil fish, such as Lepidosteus, but lack the opercula ; others are reptilian in type. In some the skull is wide, with elongated posterolateral horns, e.g. Diplocaulus magnicornis (Fig. 168), in which the orbital cavities are
Fig. 168 Dorsal View of Skull of Diplocaulus magnicornis Texas. (After O. Abel.) Ad.n. : lacrimal (adnasal). PF. : parietal foramen
D:So. : posterior parietal. Pmx. : premaxilla.
Fr. : frontal. Po. Fr : postfrontal.
jugular. Po. Orb. : postorbital.
prefrontal or lacrimal. Qj. : quadratojugal.
nasal. Smx. : maxillary,
external nostril. Sq. : squamosal,
orbit. Tab. : tabular.
(From E. S. Goodrich.)
close together on the top of the head, and a relatively small parietal foramen lies some distance behind them. In other cases (Fig. 169) the skull is narrow, as in Dolichosoma longissimum, found in the lower Permian, Bohemia. In this the parietal foramen is of moderate size and the frontal and parietal bones are fused in the median plane, and are partially fused with each other, so as to form a fronto-parietal plate, as in living types. In the primitive type Branchiosaurus amblystomus and in Protriton (Fig. 170) the parietal foramen is relatively very large, and the parietal and frontal bones are separate. This is the case also in Eryops megacephalus and Capitosaurus (Fig. 171, B). A special feature of some of the primitive types such as Protriton (Fig. 170) is the presence of circumorbital bones excluding the frontal bone from participation in the formation of the orbit and a ring of sclerotic plates for the protection of the eye within the orbit.
A — Dorsal aspect of skull of Metanerpeton pulcherrimum, showing the broad type of head with separate parietal and frontal bones. (Fritsch 1 1, after Credner.)
B — Dolichosoma longissimum, showing narrow type of skull with fused parietal and frontal bones. Lower Permian, Bohemia. 31. (After Fritsch.)
ep. : epiotic.
fr. : frontal.
ju. : jugular.
mx. : maxilla.
na. : nasal.
Orb. : orbit.
pa. : parietal.
pi.f. : pineal foramen.
pmx. : premaxilla.
po. : postorbital. Po.f. : postfrontal. pr.f. : prefrontal. Qu. j. : quadratojugal. 50. : supraoccipital. sq. : squamosal. st. : supratemporal. x. : anterior.
A general consideration of the skull of labyrinthodonts in all six orders of which a conspicuous parietal foramen is present, while it appears to be generally absent as a recognizable feature in macerated skulls of modern amphibians corroborates the impression gained by the study of the parietal foramen in fishes, namely, that its presence as a relatively or actually large foramen in the skulls of the more ancient types of fossils and the tendency for it to lessen in size or disappear in the more recent types and living species indicate that the pineal organ itself was more highly evolved
Fig. 170. — Dorsal View of Skull of Protriton, one of the Smaller Stego cephala, Magnified.
branchial arches. Nar. : nostril,
frontal. P. : parietal.
• parietal foramen. Pf. : prefrontal.
maxilla. Sc. pi. : sclerotic plates.
nasal. S. oc. : supraoccipital.
(From Wiedersheim, after Fritsch.)
A — Dorsal aspect of skull of Procolophon (Seeley), a primitive reptile showing a
large parietal foramen. B — Cranium of Capitosaurus (Zittel), an extinct tailed amphibian belonging to the
order Stegocephala, or labyrinthodonts, showing a parietal foramen.
in the more ancient and primitive types, and has become vestigial in the living representatives of these types, both in fishes and Amphibia.
Before passing on to the description of the pineal system of Reptilia it will be worth while to recall some of the effects which ensue from the disuse of the lateral eyes in animals which have for many generations lived in situations where they are completely cut off from light and are unable to use the eyes as a guide to locomotion or the capture of food. The structural degeneration of the lateral eyes under these circumstances is very similar to that which has occurred in the pineal sense organs. In both cases there is a withdrawal of the unused sense organ beneath the superficial structures, a disappearance of the refractile mechanism and of the sensory or optic nerve-fibres which connect the eye with the central nervous system. Examples of these degenerative changes are found in widely different classes of animals and in both vertebrates and invertebrates. They are well known in such cave-dwelling animals as Proteus anguineus ; burrowing animals such as the mole or common earth worm, or as we have previously mentioned Balanoglossus and Amphioxus which bury themselves in the sand. Other instances occur in the deep-sea fishes which live in total darkness, 2,000 to 3,000 fathoms beneath the surface of the ocean, and there are also the parasitic worms and other animals which live imbedded in the flesh, tissues or organs of their hosts, and those which in their adult state become fixed like the barnacles to rocks or other objects, and which though possessing motile processes or cilia are incapable of locomotion and become blind.
One of the most interesting examples of such structural degeneration is the olm, or Proteus anguineus (Fig. 159, p. 225). It lives in the subterranean streams and pools of Carniolia, Carinthia, and Dalmatia, and resembles a lizard, but has external gills uncovered by an operculum ; these may be removed without causing the death of the animal, which has lungs like those of other amphibia. The colour of the animal has been described as being of a " fleshy whiteness and transparency," and in this respect as well as in the retention of its external gills it resembles the Mexican axolotls (Amblystoma tigrinum) which also retain the larval condition of their gills.
The general appearance of Proteus anguineus and the condition of the eyes have been well described by Karl Semper (1883). The eye is completely buried beneath the superficial structures, being entirely covered by the skin. It has with the exception of the lens and the adjoining ciliary apparatus and iris all the characteristic parts of the eye. The development of these parts has, however, been arrested at an early embryonic stage. Inversion of the retina has taken place, and there is a deposit of pigment in its outer layer. Notwithstanding the imperfect development of the eyes, Semper was able to prove from observations he made on a family of Proteus that he kept for four years that the creatures were " highly sensitive to diffused light," and he remarked that " as this light contains no heat rays the eye of the Proteus can receive no impression but that of light."
Semper also writes of the mole, this animal " has true eyes from which none of the essential parts of the eyes of vertebrates are absent, although these parts are all of the simplest embryonic structure. The whole eye is very small, deeply imbedded in muscles, and quite covered by the skin, so that it is quite invisible externally. The lens consists of a number of minute and little altered embryonic cells. The retina, in the same way, is much simpler than in the eyes of other vertebrates. True degeneration, then, such as makes the eye incapable of seeing, has not taken place ; nevertheless the eye of the mole is reduced to almost total inefficiency, even when by chance it has an opportunity for using it. This almost total blindness of the mole is the result of complete degeneration of the optic nerve, so that the images which are probably formed in the eye itself can never be transmitted to the animal's consciousness. Occasionally the mole can see a little, for it has been found that both optic nerves are not always degenerated in the same individual, so that one eye may remain in communication with the brain, while the other has no connection with it. In the embryo of the mole, and without exception, both eyes are originally connected with the brain by well-developed optic nerves, and so theoretically (are capable of becoming) efficient. This may be regarded as a perfectly conclusive proof that the blind mole is descended from progenitors that could see."
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