The eyelids of Fishes are little more than marginal folds of skin, capable of little if any movement, and leave the eyes largely uncovered. Some Sharks have a third eyelid or "membrana nictitans" at the anterior corner of the eye. Lachrymal glands are unknown.
Fig. 227.—The telescopic eyes of Opisthoproctus soleatus, Vaill. (A), and of a species of a new family of Teleosts from the Indian Ocean (B). Nat. size. (From Chun.)
The Parietal Eye.—It is only in the Cyclostomes that this structure can have any claim to be regarded as a visual organ. In the Lamprey (Fig. 228) the parietal eye is a slightly flattened vesicle lying directly over the pineal vesicle, and connected by a slender stalk or nerve with the right ganglion habenulae. The dorsal or more external half of the vesicle is bi-convex, and forms the "pellucida," while the inner half or retina is said to consist of supporting cells with interspersed deeply pigmented sense-cells and ganglion cells.[[472]] The external skin over the parietal eye is partially transparent in the living animal.
In many of the oldest known Fishes, such as the Ostracodermi, the Antiarchi, and the Crossopterygian Osteolepida, there are indications of the existence either of one or of two median sense-organs on the upper surface of the skull, in the shape of one or two foramina, or hollow protuberances, or pit-like grooves or depressions, but, as a rule, when one of them is present the other is absent. It is probable that both these structures were associated with sensory organs, of which one may have been a parietal eye and the other a pineal eye. Some Teleosts (e.g. many deep-sea Scopelidae) have a transparent, convex, cornea-like prominence on the upper surface of the head which may be related to one of these singular organs.[[473]]
Fig. 228.—Vertical section through the parietal eye and the pineal vesicle of Petromyzon marinus. c.t, Connective tissue; p, pellucida; p.o, pineal organ; pt.o, parietal eye; r, retina; iii v, third ventricle. (From Wiedersheim, after Studnička.)
CHAPTER XV
THE KIDNEYS AND THE REPRODUCTIVE ORGANS—BREEDING
The kidneys and the reproductive organs are so intimately connected that it is necessary to deal with them together. Both organs are specialised portions of the coelom and its epithelial lining. The kidneys are essentially a series of tubular and at first segmentally-disposed outgrowths from the coelom (urocoeles) which acquire a connexion with the exterior, while the gonads have their origin from local modifications of the coelomic epithelium. At a very early embryonic stage each lateral half of the coelom presents three well-marked divisions: (1) a series of dorsal portions ("myocoeles"), the cavities of the myotomes or muscle-segments; (2) a longitudinally continuous unsegmented portion extending round the alimentary canal, the "ventral coelom"; and (3) a series of intermediate tubular portions or "nephrotomes," each of which leads from a myocoele to the ventral coelom (Fig. 229, A). The essential components of the kidneys, the urocoeles or renal tubules, are derived from the nephrotomes. In its typical condition each kidney consists of three portions, which, in accordance with their embryological and evolutionary sequence, are termed the "pronephros," the "mesonephros," and the "metanephros." The pronephros, the larval or provisional kidney, is formed from a limited number of the nephrotomes immediately behind the head. From each nephrotome a hollow tubular outgrowth is formed, which grows towards the lateral surface of the body, and then unites with its fellows of the same side to form a main longitudinal duct—the "archinephric" or "pronephric duct" (Fig. 229, A, Fig. 230, A). This duct grows backwards until it opens into the cloaca.[[474]]