DEVELOPMENT OF NEMEBTEANS. 



197 



both the sheath and proboscis he 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 oviparoiTs 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 Nemerteans 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 

 ^dult form. In others there is a complete 

 und most interesting metamorphosis. In 

 several Nemertean worms the egg undergoes 

 total segmentation, leaving a segmentation- 

 cavity. The next occun-ence is the separa- 

 tion of a one-layered ciliated blastoderm, the 

 ectoderm, which invaginates, forming the 

 primitive digestive cavity, from which the 

 stomach and oesophagus are formed. The 



Fig 135 — Pro- 

 rhyrtchus Jiumatills, 

 one of the simplest 

 Nemerlean worms. 

 o. mouth ; m, ceso- 

 phagu8;i, iniestine; 

 gl, glands opening 

 into the intes- 

 tine; 0. ciliated pits; 

 X, style in the pro- 

 boscis 



larva (originally described under the name of above the resopha- 

 Pilidium) is now helmet - shaped, ciliated, f liu/sac atWra* 

 with a long lash (flagellum) attached to the diffeTe,.f*l"^f ft 

 posterior end of the body. (Pig. 136.) tCrexLmlS? 



After swimming about on the surface of gSur"^'"'' *^^" 

 the sea a while, the Nemertes begins to grow 

 out from near the oesophagus of the Pilidium. On eacli 



