CYCLOSTOMES. 315 



fluid the nitrogenous waste and pours it into an excretory 

 or urinary duct which empties behind, near the anus. 



The reproductive system is closely related to the excre- 

 tory organs. In all except a few fishes the sexes are sepa- 

 rate. In the females, eggs are formed in sped: 1 structures, 

 the 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 consider- 

 ably in size. In the mammals the diameter is -about - f -^ of 

 an inch, the ostrich lays an egg about 5 inches in diam- 

 eter, while the egg of jEpiornis, one of the extinct birds of 

 Madagascar, was equal in size to 150 hen's eggs. 



The Vertebrates are divided into Cyclostomes and 

 Gnathos tomes. 



SERIES 1. 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. Vertebra 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 



