CLASS OF FISHES. 381 



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. 52). 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 portse.* 



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. 49). 



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 



Fig. 391. Hippocampus or Sea Horse. 



number, such as the hippocampi, commonly called sea-horses 

 (Fig. 391), they are, on the contrary, ramified, and have the 

 form of bunches of feathers ; finally, in most cartilaginous 

 fishes, as in the skate and shark, they are attached by their 

 external edge to the skin, as well as by their internal to the 

 branchial arches. 



The mechanism of breathing is best observed in the living 

 fish : the mouth and gill-covers open alternately ; the water 

 entering by the mouth escapes by the openings of the gills, 

 the gills themselves having in the meantime been bathed in 

 it ; whilst this happens, the oxygen is extracted from the 

 water. In osseous fishes there is generally but one aperture 



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



