PHYLUM ANNUL ATA 



501 



body elongates, and the mesoderm-bands, developed as in the 

 Chaetopoda, become divided into as many as fifteen segments. A 

 circlet of setae is developed at the anal end, and subsequently 

 the two ventral setae are formed in the same manner as in 

 the Chaetopoda. The pre-oral lobe becomes narrowed to form the 

 cylindrical proboscis of the adult ; and the rudimentary segmenta- 

 tion gradually disappears as development advances. 



In Bonellia there is unequal segmentation, as in most Chaetopoda 

 resulting in the formation of four large megameres and eight small 



micromeres : the latter multiply 

 repr.ap rapidly, and grow over the mega- 



meres so as eventually to enclose 

 the latter in a complete layer of 

 ectoderm, save at one point, where 

 there is a gap^ the blastopore. Here 

 the ectoderm bends inwards to 



coel 



ves.sem 

 all 



FTG. 397. Male of Bonellia. all. 

 alimentary-canal ; ccel. groups of 

 cretonne cells destined to give 

 rise to sperms ; rtpr. ap. repro- 

 ductive aperture ; ves. sem. vesi- 

 cula seminalis. (After Greef.) 



Fin. 398, Trochophore of Echiurus. 

 an. anus ; ap. pi. apical plate ; int. in- 

 testine ; mo. mouth ; ne. co. rudiment 

 of nerve-cord ; &s. oesophagus ; cts. 

 conn, cesophageal connective ; atom. 

 stomach. (After Hatschek.) 



give rise to a continuous mesoderm layer superficial to the mega- 

 meres. The blastopore soon closes up. The megameres divide to 

 form the cells of the endoderm, among which a lumen only appears 

 comparatively late; mouth and oesophagus are developed as an 

 outgrowth, at first solid, from the endoderm. The anus becomes 

 formed still later by invagination at the hinder end of the body ; 

 and a pair of epidermal vesicles which appear at its sides, developed 

 as outgrowths from the terminal, part of the intestine, form the 

 rudiments of the posterior nephridia. A rudimentary pre-oral 

 lobe becomes established. The mesoderm remains unsegmented, 



