DEVELOPMENT OF NEMERTEANS. 



217 



lU 



■^ti 



botli tlie sheath and proboscis lie between the commissures 

 of the ganglia in the front part of the head. 



The ovai'ies 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 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 gi'owth. In still another spe- 

 cies {JVemertes 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 ]Sl"emertean worms the egg undergoes 

 total segmentation, leaving a segmentation- 

 cavity. The next occurrence is the separa- 

 tion of a one-layered ciliated blastoderm, the 

 ectoderm, which invaginates, forming 

 primitive digestive cavity, from which 

 stomach and oesophagus are formed. 



Kg. 141. — Pro- 

 rhynchus fiuviatilis, 

 one of the simplest 

 Nemertean worms. 

 0, mouth ; £e, ceso- 

 the phagu8;z, intestine; 

 </l^ glands opening 



the ^"**^ ^^^ intes- 

 tine; 0, ciliated pits; 

 The ^> style in the pro- 



larva (originally described under the name of ahove the cesopha- 

 Pilidium) is now helmet - shaped, ciliated, biin'a sac at y ,• op, 

 with a long lash (flagellum) attached to the different' stafe? if 

 posterior end of the body. (Fig. 142.) t™" rStemlii? 



After swimming about on the surface of liX'urr"^"" '^'' 

 the sea a while, the Nemertes begins to grow 

 out from near the cBSophagus of the Pilidium. On each 



