272 CHARACTERS OF VERTEBRATE ANIMALS 



gullet, stomach, and intestine, the last not opening into a 

 cloaca as in Dog- Fish, nor provided with a spiral valve (see 

 p. 261). A large liver is present, but the pancreas appears to 

 be represented physiologically by some blindly- ending tubes 

 {pyloric ccsca) which open into the beginning of the small 

 intestine. 



The Circulatory Organs, broadly speaking, are constructed 

 on the same type as in Dog-Fish (see p. 261), though important 

 differences are seen in the heart, which no longer possesses 

 an arterial cone with numerous rows of valves, but presents a 

 new structure, the arterial bulb (fig. 163), which succeeds the 

 ventricle, and is really the swollen beginning of the ventral 

 aorta, which, as before, runs forwards on the floor of the gill- 

 region, and gives off the afferent branchial arteries that carry the 

 impure blood to the gills to be purified. 



The only organs of respiration are the gills, of which enough 

 has been said to give an idea of their arrangement. 



Nervous System and Sense Organs (fig. 163). — The brain 

 presents the same regions as in a Dog-Fish (see p. 263), but is 

 shorter and broader. The olfactory lobes are not placed on 

 stalks, and there are two cerebral hemispheres instead of an 

 unpaired projection, while the cerebellum is smooth and tongue- 

 shaped. 



Two obvious points as regards the sense-organs may be 

 mentioned. One is that the nasal sacs have no internal openings 

 into the mouth-cavity as in Dipnoi, but are distinguished by the 

 possession of double external nostrils; while the other peculiarity 

 is a negative one and consists in the absence of the numerous 

 jelly-tubes so characteristic of the Dog-Fish (see p. 263). 



A membranous swim-bladder (fig. 163) is present, situated 

 above the other internal organs, close to the under surface of 

 the backbone, but though it contains air, it has lost in the adult 

 all connection with the gullet, and therefore cannot play even a 

 subsidiary part in respiration. 



Little more can be done here than briefly mention some of 

 the more important or interesting types of bony fishes, but even 

 so, it will make things clearer to indicate the chief groups, which 

 are as follows: — 



A. — Teleosts in which the swim-bladder when present has 

 lost its connection with the gullet. 



