DEVELOPMENT OF NEMERTEAN8. 



197 



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 '\t'StVt~y 



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 IkJ"^--^'?!.' 

 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 adnlt 

 form by direct growth. In still another spe- 

 cies {A'emertes communis) the embryo is a 

 ciliated gastrula, but leaves the egg in the 

 jidult form. In others there is a complete 

 iind most interesting metamorphosis. In 

 several l^emertean worms the egg undergoes 

 total segmentation, leaving a segmentation- Kg. 135 _ p^ 

 cavity. The next occun-ence is the separa- l^etfai'Smpitd 

 tion of a one-layered ciliated blastoderm, the Nemoriean worms. 



- . ^ . . ' -^^ 0. mouth ; £», ceso- 



cctoderm, which mvagmates, forming the phag"*:*, intestine; 



■ ■,- T ,• ■, J. , . V ,, ff'' glands opening 



pnmitive digestive cavity, from which the ""° U'?. intes- 

 stomach and oesophagus are formed. The ^TsVe^in'^'he "p'o- 

 larva (originally described under the name of above the ffi^^o^fm- 

 Filidium) is now helmet - shaped, ciliated, fmKrsic''att'T 

 with a long lash (flagellum) attached to the SenT'sVgi' ft 

 posterior end of the body. (Pig-. 136.1 development. The 



.J,, . . J V 6 ""•/ worm IS externally 



Alter swimming about on the surface of ciUated.-After Qe- 

 the sea a while, the Nemertes begins to grow 

 out from near the oesophagus of the Pilidium. On each 



