DEVELOPMENT OF NEMERTEAN8. 



155 



both the sheath and proboscis lie between the commissures 

 of the ganglia in the front part of the head. 



The ovaries and testes are situated in sacs 

 on each side of the digestive canal. The 

 sexes are distinct, with the exception of cer- 

 tain species of Borlasia. The breeding sea- 

 son is from March to April, while others 

 spawn all summer. The eggs are ejected 

 from lateral, pale, minute openings, and the 

 species may be either oviparous or ovovivipa- 

 rous. These worms when molested often 

 break into fragments ; in such cases each 

 piece is capable of reproducing the entire ani- 

 mal and all its internal organs. 



The Xemerteans present a great range of 

 variation in their mode of development. In 

 the simplest mode of growth the young is a 

 ciliated oval form, without any body-cavity. 

 In others there is a body-cavity, but the larva 

 is minute and ciliated, and attains the adult 

 form by direct growth. In still another spe- 

 cies (Nemertes communis) the embryo is a 

 ciliated gastrula, but leaves the egg in the 

 adult form. In others there is a complete 

 and most interesting metamorphosis. In 

 several Xemertean worms the egg undergoes 

 total segmentation, leaving a segmentation- 

 cavity. The next occurrence is the separa- ; 

 tion of a one-layered ciliated blastoderm, the ^"mo^h" 

 ectoderm, which invaginates, forming the 

 primitive digestive cavity, from which the 

 stomach and oesophagus are formed. The 

 larva (originally described under the name of above the 

 Pilidium} is now helmet - shaped, ciliated, ESd 

 with a long lash (flagellum) attached to the differ.: 

 posterior end of the body. (Fig. 106.) 



After swimming about on the surface of 

 the sea a while, the Nemertes begins to grow 

 out from near the oesophagus of the Pilidium. On each 



n>. 105. pro- 



testine; 



ffl, glands opening 



iuto tne intes- 



tine; o, ciliated pits; 



x, style in the pro- 



lopment. The 

 worm is externally 



