INTRODUCTORY 243 



tinually renewed. The veins which return the blood to the 

 heart all unite, with the exception of those from the liver, into 

 two transverse veins called the Cuvierian veins, which meet 

 together at their opening into the auricle ; there is no posterior 

 vena cava as in higher vertebrates. The hepatic veins open 

 directly into the sinus venosus formed by the union of the 

 Cuvierian veins. 



In the digestive organs the points of difference from those 

 of other vertebrates are not of great importance. The narrow 

 gullet is the continuation of the pharynx and opens into the 

 stomach. The short and wide intestine provided internally 

 with a spiral valve is characteristic of Elasmobranchs and the 

 valve disappears in the Teleostei. In the latter also the pan- 

 creas is reduced or wanting and its function is probably per- 

 formed by a number of short tubes opening into the beginning 

 of the intestine, and known as the pyloric caeca. In the nervous 

 system the brain is chiefly distinguished by the great develop- 

 ment of the cerebellum and the small size of the cerebral hemi- 

 spheres in most fishes ; the hemispheres are less developed in 

 Teleosteans than in Elasmobranchs, but in the Dipnoi, on the 

 other hand, they are large and well-developed. The three 

 chief pairs of sense organs, olfactory sacs, eyes, and auditory 

 organs, are present. The olfactory sacs are a pair of simple 

 depressions or invaginations of the epidermis opening in front 

 of the mouth ; the division of the opening by the dermal bones 

 of the upper jaw has already been mentioned. The eyes have a 

 flatter cornea and a more spherical lens than those of terrestrial 

 vertebrates ; in Teleosteans there is a special layer of reflecting 

 tissue, composed of a chemical compound called guanin, behind 

 the retina. The auditory organs are membranous sacs entirely 

 enclosed in the auditory capsules of the skull, each is divided 

 into an upper part provided with the usual three semicircular 

 canals and a lower part called the sacculus ; opening into the 

 latter is a narrow tube which is the vestige of the epidermic 

 invagination by which the auditory sac is formed in develop- 

 ment ; for the organ of the sense of hearing like that of the 

 sense of smell is nothing but a portion of the external skin, 

 which in the development of the embryo becomes pushed in to 

 form a hollow bladder ; the bladder or sac becomes complicated, 

 and the tube by which it originally opened to the surface 



