os 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 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. 
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. Cillary 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.” 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. 
