318 



BRANCH CHORDATA 



are especially developed and are connected by the corpus cal- 

 losum, which is not present in birds. The brain (Fig. 259), with 

 a few exceptions, is convoluted. 



The special senses are all present except in some Cetacea, 

 where the olfactory membrane is degenerate and the sense of 

 smell is lacking. The organs of sight and hearing (Fig. 260) 

 vary most, owing to the different environments and consequent 

 habits and needs of the various species. The sense of touch, 

 while distributed over the surface of the body, is especially 



Fig. 260. — Diagram of the labyrinth of the ear in /, the fish; //, the bird; 

 and ///, a mammal: U, Utriculus; D, sacculus; US, utriculus and sac- 

 culus; Cr, canalis reuniens; R, recessus labyrinthi; UC, commencement of 

 the cochlea, C, L, lagena; K, cecal sac at the apex; C, cecal sac of the ves- 

 tibulum of the cochlear canal. (After Waldeyer, from Gegenbaur.) 



sensitive at the ends of the fingers, on the lips, tongue or snout, 

 and, in some monkeys, upon the under surface of the tail. The 

 cat has long sensitive hairs (vihrissw) connected with nerve-end- 

 ings, which are tactile in function. The sense of taste, situated 

 on the base and tip of the tongue and on the soft palate, is more 

 highly developed than in any other class. 



The sense of scent or smell is highly developed. Correlated 

 with the development of smell is the presence of odoriferous 

 glands in many mammals. The odors may serve for recognition, 



