390 MANUAL OF ZOOLOGY SECT. 



continuous pavement of smooth plates covered with 

 enamel, adapted to crushing food consisting of such objects 

 as Shellfish and the like. 



The various divisions of the enteric canal are similar in 

 all members of the class to what has already been described 

 in the case of the Dog-fish. A spiral valve is always 

 present in the large intestine, though its arrangement 

 varies considerably in the different families. The rectum 

 always terminates in a cloaca into which the urinary and 

 genital ducts also lead. 



The respiratory organs have in all the same general 

 arrangement as in the Dog-fish. The inter-branchial septa 

 are of considerable breadth and the gill-filaments are 

 attached to them along their entire length. 



The heart also has in all essential respects the same 

 structure throughout the group : the most characteristic 

 feature being the presence of a conus arteriosus which is 

 rhythmically contractile and contains several rows of valves. 



Impregnation is internal in all the Elasmobranchii with the 

 exception of the Greenland Shark (Laemargus\ the claspers 

 acting as intromittent organs by whose agency the semen is 

 transmitted into the interior of the oviducts. In all the ova 

 are very large, consisting of a large mass of yolk with a 

 disc of protoplasm the germinal disc, on one side. The 

 ripe ovum ruptures the delicate wall of the follicle in which 

 it is enclosed, and escapes into the abdominal cavity to 

 enter one of the oviducts, as already stated in the case of 

 the Dog-fish. Impregnation takes place in the oviduct, and 

 in the oviparous forms the impregnated ovum becomes 

 enclosed in a horn-like shell secreted by the shell-gland. 

 Enclosed in the shell, the form of which varies in 

 different groups, the egg passes to the exterior, and 

 undergoes development until the young fish is fully formed, 



