xii PHYLUM CHORDATA 393 



or triangular with serrated edges, or made up of several 

 sharp cusps; in the rays, however, the teeth are more or 

 less obtuse, sometimes, as in the eagle rays, forming a con- 

 tinuous pavement of smooth plates covered with enamel, 

 adapted to crushing food consisting of such objects as 

 shell-fish 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 dogfish. A spiral valve is always present 

 in the large intestine, though its arrangement varies con- 

 siderably in the different families. The rectum always ter- 

 minates 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 dogfish. 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 (Lcemargus}, the 

 claspers acting as intromittent organs by whose agency the 

 semen is transmitted into the interior of the oviducts. In 

 all Elasmobranchs the ova are very large, consisting of a 

 large mass of yolk with, on one side, a disc of protoplasm, 

 the germinal disc. 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 dogfish. Impregnation takes place 

 in the oviduct, and in the oviparous forms the impregnated 

 ovum becomes enclosed in a chitinous shell secreted by the 



