SENSE-ORGANS 393 



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 iisually 

 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. 1 



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," consist- 

 ing 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 " pro- 

 cessus 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. 2 Some oceanic pelagic Teleosts are remarkable 

 for their curious telescopic eyes in the shape of short protruding 



1 Kyle, Journ. Linn. Soc. (Zool.). xxvii. 1900, p. 541. 



2 Beer, Wien. klin. Wochenschr., No. xlii. 1898, p. 11. 



