150 ELEMENTS OF COMPARATIVE ZOOLOGY. 



ovaries, and when ripe the eggs are passed out to the 

 exterior by means of a tube (oviduct) developed from the 

 urinary duct. This passage may be rapid, or the egg may 

 remain for a time in the oviduct and there undergo its 

 development, as is the case in certain members of all groups 

 of vertebrates except birds. 



In the male, corresponding to the ovaries in position, 

 etc., are the testes, which produce the male reproductive 

 element, which is also carried off by a part of the primitive 

 excretory duct. 



All vertebrates produce eggs, but these vary considerably 

 in size. In the mammals the diameter is about T ^ of an 

 inch, the ostrich lays an egg about 5 inches in diameter, 

 while the egg of one of the extinct birds of Madagascar 

 was equal in size to 150 hen's eggs. 



The Vertebrates are divided into Cyclostomes and 

 Gnathostomes. 



CYCLOSTOMATA. 



The Cyclostomes include a few eel-like forms, commonly 

 known as lampreys and hagfishes. These differ from the 

 other Vertebrates in many points, some of which are 

 mentioned here. Bone is entirely lacking, and cartilage 

 is feebly developed. Vertebrae are scarcely recognizable, 

 and there are no traces of paired fins, although dorsal and 

 caudal fins may occur. The mouth, as the name Cyclo- 

 stome implies, is circular, but is incapable of closure like 

 that of other vertebrates, since movable jaws are lacking. 

 Inside of the mouth are horny teeth (few in the hagfishes, 

 many in the lampreys), but these are chiefly used for 

 holding, not for biting or crushing. The tongue is very 

 large. 



There is but a single nostril on top of the head, The 



