80 



ANIMAL STUDIES 



covered with cilia, create the currents of water which 

 bring minute organisms serving as food within reach of 

 the mouth. 



75. Development. Unlike the earthworms, the Poly- 

 chsetes lay their eggs in the sea water, where they are left 

 alone to develop as best they may. Both the male and 

 female Nereis, as the egg-laying time approaches, undergo 

 remarkable changes in their external appearance, resulting 

 in the form shown in Fig. 42, A. 



They are now active swimmers, and 

 thus are able to scatter the fertilized 

 eggs over wide and more or less favor- 

 able areas. The young also for a 

 time are free-swimming, but finally 

 end their migrations by settling to 

 the sea bottom, where they gradually 

 attain the adult condition. 



As in some of the flatworms, re- 

 production may also occur asexually 

 by the division of the animal into two 

 or more parts, each of which subse- 

 quently becomes a complete indi- 

 vidual. In other species growth of 

 various parts may result in two com- 

 plete worms at the time of separation ; 

 and from such forms we may trace a 

 fairly complete series up to those in 

 which the original parent breaks up Fw.48. 



. , , , , , , . , la). Right-hand figure il- 



mto twenty to thirty young. lustrates a]imentary canal . 



76. The leeches. - At first Sight 



the leeches (Fig. 46), or at least the 



smaller, more leaf-like forms, might 



be mistaken for flatworms, especially for some of the para- 



sitic species. As in the latter, the mouth is surrounded by 



a sucker, and another is located at the hinder end of the 



body, but beyond this point the resemblance ceases. The 



pharynx ; c, crop ; p, 



pouches ; ''" in 



