336 



ZOOLOGY. 



lives. Their heart is composed of two cavities, the auricle 

 and ventricle, containing only dark blood ; this blood is sent 

 to the gills, and returns from these after being exposed to 

 the oxygen, to be distributed to the various parts of the 

 body, no heart being interposed between the gills and the 

 other organs of the body ; their blood is cold, and their skin 

 naked, covered only with scales; they lay eggs, that being 

 their mode of reproduction ; and finally, their limbs have the 

 form of fins. 



475. They differ considerably in the form of their 

 bodies, but the outline is generally simple, there is no neck, 



Sroperly so called, and the head is large; their tail is not 

 istinguishable from the rest of the body. Some have no 

 fins, but generally we find them present, arranged in pairs 

 symmetrically at the sides or singly on the back and abdo- 



r/2 



Fig. 306.* 



men (Fig. 306). Those in pairs represent the limbs of ver- 

 tebrate animals. The anterior are called pectoral fins ; the 

 inferior, which vary much in position, are called ventral ; the 

 asymmetrical fins are the dorsal (d), anal (a), and caudal (c), 

 according as they are placed on the back, under the tail, or at 

 its extremity. They resemble each other in structure, and 

 consist almost always of a fold of skin, supported by . 

 or cartilaginous rays, pretty much in the way that the wings 

 of bats and dragons are supported by the fingers and ribs. 



The large openings leading to the gills are placed behind 

 the head ; in breathing, the water passing into the mouth is 



Le Rouget (MullusBarbatus), the Barbed Mullet, to show the different 

 fins, Ac.: p, pectoral fin; c, ventral fin; </l, first dorsal; </.'. 

 dorsal; r, caudal; a, anal; o, opening leading to the gills; 6, feelers, or barbs 

 of the lower Jaw. 



