PHYLUM ANNULATA 



501 



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

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

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

 the two ventral set;e 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 Chajtopoda 

 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 hlastofore. Here 

 the ectoderm bends inwai'ds to 



t 



-coel 



-ve!?.sera 

 -ali 



a/j// 



Uo I 



Fio. 3!i7.— Male of Bonellia. ali. 

 alimcntiiry-canal ; c(xl. gi-oi;ps of 

 nelumic cells destined to give 

 rise to sperms ; rejjr. ap. repro- 

 diietive aperture ; vch. .*(-//?. vesi- 

 cula seminalia, (After Grecf.) 



Fki. sns.— Troehophore of Echiums. 

 aa. anus ; ((/>. i>U apical plate ; iat. in- 

 testine ; ^tiQ, niouth ; ne. co. rudiment 

 of nerve-cord ; o's. oesophagi s ; (es. 

 eonn. oesophageal connective ; &ioni. 

 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, 



