IV. VERTEBRATA; PISCES. 



499 



called cerebrum being only the corpora striata (BC). The independent ol- 

 factory lobes lie either close to the cerebrum (most teleosts,Lo/) or are separated 

 from it by an olfactory tract (fig. 549, Lol). The optic thalami are small (d), 

 but below them are enlargements characteristic of iishes, the lobi inferiores, and 

 between them the saccidus vasadosiis. Both optic lobes and cerebellum are 

 greatly developed. 



The nose consists of two preoral pits, the opening being divided by a bridge 

 of skin into anterior and posterior nostrils. In many selachians the nostrils 

 are connected with the mouth by a oronasal groove covered by a fold of skin, 

 and in tlie Dipnoi there is a choana. The eye has several peculiarities. The 



Fig. 54S. Fig. 549. 



Fio. 548. — Brain of trout (after Wiedersheim). BG, corpus striatum; CP, epiphy- 

 sis; HH, cerebenum;io/, olfactory lobes; I\IH, optic lobes; NH, medulla oblongata; 

 Pall, pallium, in part cut away; VH, cerebrum; I-XII, nerves. (See p. 471). 



Fig. 549. — Brain and nasal capsules of Scylllttm catuhis (from Wiedersheim). /r, 

 fossa rhomboidalis; tr, olfactory tract. 



lens is very convex, almost conical, and the eye is very short-sighted because 

 light is so absorbed by water that objects forty feet away are invisible. With 

 this is connected the campanula Hallcri. The processus falciformis, a sickle- 

 shaped outgrowth of the chorioid, e.xtends from the entrance of the optic nerve 

 into the vitreous body as far as the lens, swelling out into the campanula; this 

 contains a muscle which draws back the lens and so is an apparatus of accom- 

 modation. Chondrifications and ossifications of the sclera are common. Lids 

 are weakly developed or absent, and only some elasmobranchs have a nictitating 

 membrane. 



The ear has a relative size found in no other vertebrates, the labyrinth (fig. 

 532) has the sacculus and utriculus separated, the sacculus with a diverticulum, 

 the lagena, the beginning of a cochlea. In the labyrinth of the teleosts there are 

 two large otoliths. Experiments show that the ear is especially an organ of 

 equilibration, and in some an organ of hearing to a limited extent. A few 

 species have the power of making a noise, usually by the rubbing of parts on 

 each other. 



Of all sense organs of the skin, the most noticeable are those of the lateral 

 line, which occur elsewhere only in cyclostomes and aquatic amphibia. In 

 fishes a line on either side usually begins at the tail and extends to the head, where 



