tchthyopsida 



119 



than the vertebrate nervous system. The brain of the iish is differenti- 

 ated into five regions which, beyond question, correspond structurally 

 and, at least in a general way, functionally, to the five main divisions 

 of the human brain (Fig. 324). In the telencephalon, however, cerebral 

 hemispheres are poorly developed, and their roof, the pallium, con- 

 tains little or, in most fishes, no nervous tissue. In modern fishes, there 

 are 10 pairs of cranial nerves. The autonomic nervous system is well 

 developed. 



The "five senses" of man (but in reality he has several more than 

 five) are provided for in fishes by organs which differ only in details 

 and in degree of efficiency from corresponding organs in mammals. 

 Eyes, ears, and olfactory (nasal) organs are well developed in 

 fishes. In the mouth are organs of taste, but — unlike anything found 

 in Amniota — sense-organs similar to the oral organs of taste commonly 

 occur on the external skin, especially on the head and sometimes far 

 back on the trunk and even on the fins (see p. 196). Apparently many 

 fishes may become aware of the proximity of food by "tasting" it 

 through the skin. The skin is also well supplied with tactile sensory 

 structures but in a limited variety of types as compared to the skin of 

 land animals. 



In addition to these external organs of chemical and tactile sense, 

 fishes have an integumentary sensory equipment which has no exact 



Fig. 324. Sagittal section of brain of trout, (aq) Aqueduct; (bo) bulbus olfac- 

 torius; (ca, ch, ci, cp) anterior, horizontal, inferior, and posterior commissures; 

 (cc) central canal; (cl) cerebellum; (cs) corpus striatum; (h) hypophysis; (i) infundib- 

 ulum; (li) inferior lobe; (oc) optic chiasma; (p) pallium; (pi) pinealis; (sv) saccus 

 vasculosus; (//) torus longitudinalis; (to) tectum of optic lobes; (v) velum trans- 

 versum; (v 3 , v A ) ventricles; (vc) valvula cerebelli; (I) olfactory nerve; (IV) trochlear 

 nerve. (After Rabl-Riickhard. Courtesy, Kingsley: "Comparative Anatomy of 

 Vertebrates," Philadelphia, The Blakiston Company.) 



