CLASS OF FISHES. 



357 



scribed. The gills are fixed at both edges, and instead of 

 having one opening by which the water escapes, there are 

 several, as many openings, in fact, as there are intervals 

 between the gills. The openings are almost always external, 

 nevertheless they terminate sometimes in a common canal, 

 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 



Fig. 330. The Sturgeon. 



fishes differ much from each other, and constitute two orders 

 the selaciau (plagiostoma) and the cyclostoma. 



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

 posed of the sharks properly so called, the requin (Fig. 317), 

 the hammer-headed shark (Fig. 332), the saw-fish, &c. ; and 

 the family of the skate, in which the torpedo (Fig. 320) 

 as well as the skate properly so called have a place (Fig. 331). 

 All these fishes have five branchial openings on each side of 



Fig. 331. Skate. 



Fig. 332. Hammer-headed Shark. 



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. 317). Some are 

 oviparous, whilst others lay eggs covered with a hard and 

 horny case. 



505. The order of cyclostomes is characterized by the 

 singular conformation of the mouth, adapted only for sue- 



