Fig. 224.—Two stages in the development of the olfactory organ and the pituitary involution in Petromyzon. A is the earlier, B a much later stage. br, Brain; in, infundibulum; l.lp, lower lip; ms, mesenteron; n, notochord; ol.o, olfactory organ; pn, pineal body; pt.s, pituitary sac; st, stomodaeum; u.lp, upper lip. (From Parker and Haswell, after Dohrn.)
The Cyclostomata are unique amongst Craniates in the apparently unpaired condition of the olfactory organ, and in its remarkable relation to the pituitary involution. In the embryo Lamprey the median and ventral olfactory pit is carried inwards with the pituitary invagination, so that the former appears as a dorsal outgrowth from the latter, and the two have a common external opening, the naso-pituitary aperture (Fig. 224). Later the extraordinary forward growth of the upper lip to form the roof of the buccal funnel has the effect of shifting the naso-pituitary involution and its aperture to a final position on the dorsal side of the head. It is due to this dorsal displacement that, as we shall see, the pituitary caecum reaches the ventral surface of the brain by perforating the basis cranii from above, instead of from below as in all other Craniates. The pituitary body is pinched off from the dorsal side of the naso-pituitary involution. In the adult Lamprey the olfactory organ appears as a round sac divided by a median septum into two lateral chambers (Fig. 225), the lining epithelium of which is raised into prominent ridges. Behind the sac the pituitary involution is prolonged backwards beneath the brain, and, after traversing the basi-cranial fontanelle, it widens out into a spacious cul-de-sac and terminates on the dorsal side of the pharynx, beneath the anterior end of the notochord. In Myxine the pituitary involution ends by opening into the pharynx.
Fig. 225.—Side view of the brain of Petromyzon, with the olfactory organ and the pituitary caecum in section. cblm, Cerebellum; crb.h, cerebral hemisphere; dien, thalamencephalon; f, fold in the nasal tube; gl, nasal glands; inf, infundibulum; l.gn.hb, left ganglion habenulae; med.obl, medulla oblongata; na.ap, naso-pituitary aperture; n.ch, notochord; Nv1-nv10, cranial nerves; Nv12, first ventral spinal nerve; olf.cp, olfactory capsule; olf.l, olfactory lobe; olf.m.m, olfactory mucous membrane; opt.l, optic lobe; pn, pineal body; pn′, inferior pineal body; pn.e, parietal eye; pty.b, pituitary body; pty.p, pituitary cul-de-sac; sp, median septum of the olfactory sac; sp1, first dorsal spinal nerve. (From Parker and Haswell, after Ahlborn and Kaenische.)
The apparently monorhinal condition of the Cyclostomes is probably a secondary acquisition. At the earliest embryonic stage at which any trace of an olfactory organ is apparent, there is a median thickening of the epidermis, possibly a vestige of some older sensory organ comparable, it may be, to the so-called olfactory organ of Amphioxus; on each side of it there is a lateral thickening, the rudiments of the paired organs.[[465]] The three thickenings, or "plakodes," then sink inwards to form an olfactory pit. The partial subdivision of the adult organ by a vertical septum, and the presence of two olfactory nerves, point to the same conclusion.[[466]] All Fishes possess olfactory organs which are obviously paired. In Elasmobranchs and Dipnoi they retain their primitive ventral position. Many Sharks and Dog-Fishes possess an oro-nasal groove leading from each olfactory organ to the corresponding angle of the mouth. The Dipnoi proceed a stage farther, and, by the conversion of the grooves into short canals, the olfactory pits communicate with the mouth by true internal nostrils, as in the higher Vertebrates. In the adults of existing Teleostomi the orifice of each organ is usually divided into two by the growth of a fold of skin across it, and the two apertures rotate outwards and upwards on to the lateral or the upper surface of the snout. Of the two nostrils the posterior one probably corresponds to an external nostril, and the anterior one to the internal nostril. Occasionally each olfactory organ has only a single orifice. In the Crossopterygii and in some Teleostei the nostrils become tubular. The lining epithelium of the olfactory pits is usually produced into ridges, disposed longitudinally or transversely, or in the form of radii from a central point in the roof. Many Teleosts have each olfactory organ prolonged backwards into one or two sacs, the nasal sacs, which are either simple reservoirs, or glandular and mucus-secreting. In a species of Chinese Sole (Cynoglossus semilaevis) the two sacs, one from each olfactory organ, unite over the roof of the mouth in a common median sac, and in one unique specimen the latter communicated with the mouth by a large naso-pharyngeal aperture.[[467]]
The Eyes.—In essential structure the eyes of Cyclostomes and Fishes resemble those of the higher Craniates. As a rule, in Fishes they are relatively larger, however, and the lens is globular and the cornea somewhat flatter. Ciliary processes and ciliary muscles are absent. As the eyes are nearly always lateral in position it is probable that monocular vision is the rule. In Teleosts and in Amia a "choroid gland," consisting of a mass of capillary blood-vessels, surrounds the optic nerve externally to the retina, and derives its blood from the efferent artery of the pseudobranch (Fig. 226). In most Teleostomi, but not in Cyclostomes, Elasmobranchs, and Dipnoi, there is a singular prolongation of the choroid coat, known as the "processus falciformis," which extends across the vitreous humour to the inner face of the lens, where it ends in an expansion, the "campanula Halleri" (Fig. 226). Accommodation to vision at different distances is not effected by alterations in the convexity of the lens, but by a change in its position with regard to the retina, apparently brought about by the contraction of a special retractor muscle.[[468]] Some oceanic pelagic Teleosts are remarkable for their curious telescopic eyes in the shape of short protruding cylinders, each terminating in a strongly convex cornea (Fig. 227).[[469]] The eyes are directed either upwards or forwards, and, as their long axes are parallel in either position, it is probable that these Fishes are capable of binocular vision. In the young of certain Teleosts occurring in the Antarctic and Indian Oceans the large eyes are situated at the extremities of extraordinary long stalks extending from the sides of the head.
Fig. 226.—Vertical section of the eye of Salmo fario (semi-diagrammatic). arg, Argentea; ch, choroid; ch.gld, choroid gland; cn, cornea; cp.hal, campanula Halleri; ir, iris; l, lens; opt.nv, optic nerve; pg, pigmentary layer; pr.fl, processus falciformis; rt, retina; scl, sclerotic. (From Parker and Haswell.)
In the quasi-parasitic Cyclostome, Myxine, and in many Teleosts belonging to widely different families, which live at great depths in the sea or inhabit subterranean waters, the eyes suffer from disuse and degenerate in structure. The influence of a deep-sea habitat on the eyes of Fishes is somewhat varied. The eyes are often small. A few abyssal Fishes are totally blind, and no external trace of eyes can be seen (Fig. 430). In such Fishes compensation is often afforded by an extraordinary development of tactile organs in the form of long barbels, or of trailing filaments derived from the median or the paired fins (Fig. 371, B). Many deep-sea forms possess eyes of the normal size, or even exceptionally large eyes, probably because either they occasionally migrate towards the surface, or else they possess phosphorescent organs and are able to see by the aid of the light they themselves emit. A blind Siluroid (Amiurus nigrilabris) frequents the cave streams of Pennsylvania, and many members of the same family which live in muddy waters have very small or even minute eyes. One of the Gobies (Typhlogobius),[[470]] which buries itself in the sand, or is found under stones in the holes of a burrowing Crab on the coast of California, is also blind. Amongst other blind Fishes Amblyopsis and Typhlichthys (Amblyopsidae)[[471]] and Lucifuga (Zoarcidae) may be mentioned, the first two inhabiting the cave streams of North America, while the third has a similar habitat in Cuba. When the eyes degenerate they dwindle in size and recede from the surface. The lens and the iris wholly or partially disappear, and although it is generally recognisable the retina loses certain of its characteristic layers, or the latter are but imperfectly formed. In Myxine even the eye-muscles are absent.