Book - The Pineal Organ (1940) 10
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The Pineal Organ - Eyes of Invertebrates
Chapter 10 The Eyes of Annulata
The phylum Annulata includes the earthworms ; fresh-water worms ; marine annelids ; the primitive Archi- Annelida (Polygordius) ; certain parasitic and tube- worms, and the leeches.
Fig. 62. Anterior part of the nervous system of Nereis, showing two pairs of eyes, ey. 1 , ey. 2 ; which are connected by short stalks, the optic nerves with br., the supraoesophageal ganglion or brain ; the brain is connected by two nerve cords, the oesophageal connectives, ces. c, with the infra-oesophageal ganglion, these pass ventrally one on each side of the oesophagus; inf.o.g., the infraoesophageal ganglia, are continuous behind with the ventral chain of fused bilateral ganglia, and nerve cords, which supplies the viscera and posterior segments of the body. (After Quatrefages — Parker and Haswell.)
The eyes of these different classes of " ringed worms " are mostly of the simple upright type ; and when present in the adult animal are mostly borne on the anterior segment of the head (prostomium) ; they are usually paired and near the median plane. The eyes in some species are exceptional in position and also in structure. Thus among the Polychaete worms, Polyophthalmos, in addition to the prostomial eyes, has pairs of luminous organs on many segments of the body ; while Leptochone has a pair on each segment and in Fabricia there is a pair on the anal segment. Moreover, in many species of Sabella and all the species of Dasychone, eyes are present on the branchial filaments. These are highly differentiated compound eyes. The eyes of leeches are peculiar in structure, being formed of a number of large refractile cells lining a tubular pit and surrounded by a pigment layer. Moreover, the nerve fibres and cells, instead of being situated on the outer side of the pigment layer, are on the inner side of the refractile cells and lie in the central axis of the tube in what appears to be its potential cavity (Fig. 66). Eyes are absent in many of the adult animals, e.g. the earth-worms and certain of the parasitic and fixed forms, such as Terebella and Serpula. The free-swimming larvae, however, in the trochophore stage are in some species characterized by the presence of one or two eye-spots, placed near the apical plate, as in the trochophore larvae of some of the echinoderms and many of the arthropods and molluscs.
Fig. 63. — Section through One of the Eyes of Nereis, showing a Vitreous, Non-cellular Type of Lens, which is continuous with the Vitreous Body contained in the Optic Cup. (After Andrews.) co.: cornea. rd. : layer of rods.
cut. : cuticle. p. : pigment.
/. : lens. re. : sensory cells of retina.
Taking Nereis, a marine worm, as an example of the class Polychaeta, or worms with many bristles, we find a distinct head bearing on its dorsal aspect two pairs of eyes. Each of these is connected with the superior oesophageal ganglion or brain by a separate optic nerve (Fig. 62). The eyes lie near the median plane, and consist of a cup-shaped vesicle closed superficially by a smooth cornea (Fig. 63). This is formed superficially by a non-cellular layer — the cuticle — which is continuous circumferentially with the surrounding cuticle ; beneath this is a layer of flattened hypoderm cells. The optic cup is filled with a viscous, non-cellular material — the vitreous. This is divided by a constriction at the mouth of the cup into a superficial lens-like structure, lying outside the cup, immediately beneath the cornea, and the true vitreous which occupies the cavity. The wall of the cup is formed by tall columnar cells, the inner ends of which are clear and rod-like. These converge towards the centre of the vitreous. The middle zone of the wall of the cup is deeply pigmented, while externally are seen the nuclei of the cells and their tapering outer ends, which are continued into a nerve-fibre layer and the optic nerve. The mouth of the cup is constricted so as to form a pupil of the immovable type, there being no iris or pupillary muscles. No special ocular muscles are developed around the eyes and there would thus be no independent movement of the eyes, the separate fields of vision would overlap and the brain would receive impulses from a combination of two binocular fields of vision.
The larva of Nereis passes through a trochophore stage in which a typical " apical plate " is formed, beneath which is the rudiment of the cerebral ganglion. In the dome-like, upper segment of the trochophore near the apical plate a pair of pigment spots, the larval eyes, are developed. These are succeeded at a later stage by the appearance of another pair.
An interesting feature in the further development of N. dumerilii is its conversion by metamorphosis into a second form called Hetero-nereis, in which one of the principal changes is a great increase in the size of the eyes. This is associated with a change in the habits of the animal, from a slow, creeping life at the bottom of the sea to one in which it swims actively through the water.
In a recent communication by R. S. Brown to the R. Soc. Edin. on the anatomy of the Polychaete Ophelia cluthensis, he describes the eyes in this species as being three in number (Fig. 64) : two anterior, right and left, and one posterior on the left side, the right member of the posterior pair being absent. He also notes that the eyes are imbedded in the substance of the brain. They are slightly flattened spheres about 20/x in diameter, and consist of a lens surrounded by a large number of closely aggregated pigment granules. The worms examined were found in a limited area in fine-grained sand, occupying the upper third of the neap-tide range, in a belt which crossed the outflow of a small stream into Karnes Bay.
A comparison of these with other Polychaete worms indicates that, as in other animals which burrow in the sand or earth, there is a tendency for the eyes to degenerate and be reduced in number. Thus in Polynoid and Syllid, two pairs of eyes are visible on the prostomium ; in Eunice and Phyllodoce one pair ; and in Nephthys and Trophonia the eyes are absent altogether (Fig. 64).
No eyes are present in the earthworm (Lumbricus), but these worms have been shown to be sensitive to bright light, the sensation being perceived by the agency of large epidermal cells which are devoid of pigment.
The commensal, parasitic, and tube-forming chaetopods, as previously mentioned, also have no eyes. Among the latter, however, there are certain notable exceptions. Thus, Andrews ' has described in certain of the tube-forming Polychseta eyes of a highly differentiated compound type ; these he found on the branchiae of Potamilla, and he considered that they might give some aid to the interpretation of arthropod eyes. In the species P. reniformis the number of eyes may be as many as seven or eight on each branchia. This animal lives in a leathery tube which may be found projecting from holes in the shells of Gasteropods and from bivalve shells. At the end of the tube the cephalic branchial plumes are expanded in a circular series supported by radiating stems, each bearing two rows of branchial filaments, all of which are directed anteriorly in the fully expanded condition. In this state the eyes are on the outer or posterior sides of the main stems of the branches, there being a row of three to eight on each of the twenty radiating stems. Each eye appears as a convex, hemispherical protuberance on the outer or convex side of the main branchial stem. The diameter of one of these eyes is about 92/x, but it varies much ; smaller eyes being often found towards the tip or even interpolated between some of the larger ones along the stem. Their colour is a uniform dark red, but in strong sunlight the reflected light is golden yellow. On section the eye is seen to be covered by a smooth cuticle continuous at the base with the surrounding cuticle ; beneath this the cells forming the eye are likewise seen to be continuous at the base with the hypoderm cells surrounding it. The cells in the centre are greatly elongated, and are differentiated into sensory cells and pigment cells. The outer ends of the sensory cells are expanded by a refractile vase-shaped inclusion, the " crystal cone." The inner end contains an oval nucleus and tapers into a delicate process which comes into close relation with branches of the two longitudinal nerves of the branchial stem, but a direct continuity of the branches could not be demonstrated with certainty. Transitional hypodermal cells are present around the base of the eye, which show the continuity of the retinal type of cell with the hypoderm cells. Andrews examined several other allied species, including Sabella microophthalmia and Hypsicomus, and he mentioned that the compound eyes of Branchiommi Kollikeri are so sensitive to light that the movement of a hand in the air at a distance of a metre from the water containing the animals would cause all the animals to withdraw into their tubes as soon as the shadow fell upon them.
1 E. A. Andrews, "Compound Eyes of Annelids, T. Potamilla" J. Morph., 5, 271, 1891.
Fig. 64. — Eyes of Polychtete Worms, showing Variations in the Number
of Ocelli. A, B- Lateral and dorsal views of the brain and eyes of Ophelia cluthensis. (After
R. S. Brown.) C, D, E — Heads of Polynoid, Eunice, and Nephthys (Cambr. Nat. Hist.). In the
species Ophelia cluthensis the right member of the posterior pair of ocelli
is absent. In Eunice only one pair is present, and in Nephthys the eyes are
br. : brain. n. tr. : nuchal lobe.
c. : cirri. ce. co. : oesophageal commissure.
c. 2 : cirrus of first body segment. p. : palp.
el. : point of attachment of elytron, per. st. : peristomium.
l.a.e. : left anterior eye. pr. st. : prostomium.
/. ant. : lobus anterior. r.a.e. : right anterior eye.
l.p.e. : left posterior eye. t. : tentacle.
n. gr. : nuchal groove.
Fig. 65. — Head of Hirudo Medicinalis, showing the Continuity of the Five Pairs of Eyes, with the Rows of Segmental Papillae.
E l to E 5 : pairs of eyes. s.p. : segmental papillae.
(From A Textbook of Zoology : Parker and Haswell.)
Fig. 66. — Section of an Eye of a Leech. c. : cuticle. o.n. : optic nerve. (From Lang's Comparative Anatomy.} gl. : glandular cell. p. : pigment. hyp. : hypodermis. r.c. : refractile cells. n.c. : nerve cells.
The Eyes of Leeches. — The general structure of the paired eyespots of leeches has already been alluded to, and it will only be necessary to mention here that in Hirudo medicinalis (Fig. 65) there are five pairs placed symmetrically in series on the dorsal aspect of the anterior sucker ; and that their arrangement in series with the sensory papilla; which occupy the same position behind them indicates that they are formed as special modifications of these. The papillae are arranged in rows extending backward from the head to the tail-end of the body. Two of these rows are placed one on each side near the median plane. These are continuous in front with the most anterior pair of eyes on the 1st segment. The other papillae on the body are arranged serially in groups of three on the dorsolateral aspect and on the middle ring of each successive segment. The innermost row of these is continuous with the eye-spots from the 2nd to the 5th segment. The number of eyes is subject to considerable variation in different species. They may be developed on the posterior sucker or may be absent altogether.
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Cite this page: Hill, M.A. (2021, May 7) Embryology Book - The Pineal Organ (1940) 10. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Pineal_Organ_(1940)_10
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