THE LOWER VERTKBKATA. 187 



The resemblance in distribution of V., VII., IX., and X. is 

 singular. Each has a ventral division which forks over 

 mouth, spiracle, or gill-slit, and each (except IX.) has a 

 dorsal sensory branch to the peculiar sense-tubules. Evi- 

 dently X. is really at least four metameric nerves partially 

 united : does its extension to the stomach mean that more 

 gill-slits once existed in that direction ? And does the 

 relation of V. to the mouth mean that that opening is a 

 pair of anterior gill-slits united together ? To discuss such 

 questions will be beyond the scope of this work, but we 

 may say that the tendency of morphologists to-day is to 

 answer the second question in the negative. 



17. Sense-organs. The olfactory organs are a pair of 

 spheroidal sacs, open ventrally, and having a great number 

 of vertical laminae projecting into the cavity from the dorsal 

 side. These serve the same object as the turbinal foldings 

 in the rabbit (chap, x., 4), viz. increase of sensitive area. 

 The olfactory sacs in fishes have no connexion with respira- 

 tion, as they have in land-vertebrates. 



The eye differs in no essentials from that of the rabbit, 

 but only in details. For example, the lens is globular, and 

 nearly fills the posterior chamber : a condition adapted to 

 seeing through water. The eye-muscles are the same as 

 before, but there are no glands, such not being needed 

 in an animal whose eyes are always in contact with water. 



The ear (fig. 97) is similar to that of the frog. It Con- 

 sists of internal ear only utriculus and sacculus are feebly 

 indicated by a slight constriction of the general sac, and a 

 little process at the posterior end is all the trace there is 

 of a cochlea. The endolymph contains "otoliths," as in 

 the frog. The semicircular canals are well developed. 



The sensory tubules of the head and lateral line have 

 already been mentioned. The resemblance of the semi- 

 circular canals to them suggests that the former may be 

 specialized members of the latter series, and that the function 

 of the tubules may be the appreciation of water- pressure. 

 But speculations on the nature of a sense we do not our- 

 selves possess are not very profitable without experimental 

 evidence. 



