CLASS OF FISHES. 345 



vertebral column, and nearly throughout its whole length. 

 Their excretory ducts terminate in a kind of bladder, the 

 external orifice of which is situated immediately behind the 

 anus and the orifice of the reproductory organs. 



Digestion seems to be performed rapidly, and the chyle is 

 absorbed by numerous lymphatic vessels which terminate by 

 several trunks in the venous s\ 7 stem near the heart. 



487. The blood of fishes is red, and the globules are 

 elliptic, and of considerable size ( 81, Fig. 28 c). The heart 

 (Fig. 43) is placed under the throat, in a cavity separated 

 from the abdomen by a kind of diaphragm protected by the 

 pharyngeal bones, the branchial arches, and the humeral 

 girdle. It is composed of an auricle and ventricle, from which 

 springs the pulmonary artery. This vessel is enlarged into a 

 contractile bulb at its commencement; it soon divides into 

 branches, which proceed to the gills ; and the blood, after 

 having traversed these organs, returns towards the heart by 

 another vessel, also passing along the edge of the branchial 

 arches. There these canals send some branches to the neigh- 

 bouring parts, and reunite to form a large dorsal artery, which 

 proceeds backwards under the vertebral column, and gives 

 branches to all parts of the body (Fig. 43). Finally, all the 

 venous blood does not pass directly to the sinus (auricle) 

 already mentioned ; that from the intestines and from some 

 other parts, before returning to the heart, circulates in the 

 liver by the vena portae.* 



Thus it is that the heart of fishes corresponds to the right 

 or anterior heart of hot-blooded animals, and the blood passes 

 only once through the heart. The circulation must be slower 

 than in them ; nevertheless, the pulmonary (branchial) circu- 

 lation is complete (Fig. 40). 



488. Respiration is performed in fishes by means of the 

 air dissolved in the water, which entering by the mouth is 

 passed across the gills or branchiae. These are vascular 

 laminae, supported by the osseous branchial arches already 

 described. Four branchiae generally exist on each side. In 

 most cartilaginous fishes there are five, and in the lamprey 

 seven. In nearly all the osseous fishes these lamellae are simple, 

 and are fixed only by the base ; but in a small number, such 

 as the hippocampi, commonly called sea-horses (Fig. 316), 

 they are, on the contrary, ramified, and have the form of 



* As in mammals, birds, &c. E. K. 



