DEVELOPMENT OF NEMERTEANS. 



19? 



*- 



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 othprs 

 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 (Xemertes 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 Nemertean worms the egg undergoes 

 total segmentation, leaving a segmentation- 

 cavity. The next occurrence is the separa- 

 tion of a one-layered ciliated blastoderm, the 

 ectodernij which invaginates, forming the S^^*^^*^ 

 primitive digestive cavitv, from which the i nto the .. in . re l s - 



tine; c, cil i a. t i*u pits; 



.stomach and oesophagus are formed. The a-, style in tn<> pro- 



/ n i -i i 1 j.1 boscls situated 



larva (originally described under the name ot above the cesoptm- 



,._;?. ,, .-.. . -. gus, which ends' in a 



Pilidium) is now helmet - shaped, ciliated, blind sac at /. or, 

 with a long lash (flagellum) attached to the different "staj 

 posterior end of the body. (Fig. 136.) 



After swimming about on the surface of " e nb t aur~ Aft 

 the sea a while, the Nemertes begins to grow 

 out from near the oesophagus of the Pilidium. On each 



Fi. 



s> - Pro- 



stae 



