CLASS OF FISHES. 



393 



by which they transmit the water externally ; finally, car- 

 tilaginous arches suspended in the flesh are often found op- 

 posite the external edges of the branchiae. Lastly, these 

 fishes differ much from each other, and constitute two orders 

 the selacian (plagiostomi) and the cyclostomi. 



Fig. 405. The Sturgeon. 



504. The first, including the family of the squali, com- 

 posed of the sharks properly so called, the requin (Fig. 392), 

 the hammer-headed shark (Fig. 407), the saw-fish, &c. ; and 

 the family of the skate, in which the torpedo (Fig. 395) 

 as well as the skate properly so called have a place (Fig. 406). 

 All these fishes have five branchial openings on each side of 

 the neck, resembling fissures ; and several have on the upper 

 part of the head two openings, called blow-holes, by which 

 the water reaches the gills when the throat is temporarily 

 filled with their prey. They are most voracious animals, 

 especially the requin or blue shark (Fig. 392). Some are 

 oviparous, whilst others lay eggs covered with a hard and 

 horny case. 



Fig. 406. Skate. 



Fig. 407. Hammer -headed Shark. 



505. The order of cyclostomes is characterized by the 

 singular conformation of the mouth, adapted only for suc- 

 tion (Fig. 408) ; they are the most imperfect of all the 

 ordinary vertebrata. Their skeleton is sometimes mem- 

 branous, as in the ammoccetes or river lampreys, and is always 

 less complex than in other fishes ; their nervous system is 

 but little developed, and the gills have the form of small bags. 



