PHYLUM ANNULATA 



483 



lobe with pre-oral and post-oral circlets of cilia, and in 

 other respects closely resembles the trochophore embryo of 

 a Chaetopod. The posterior part of the 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 setse are formed 

 in the same manner as in the Chsetopoda. The pre-oral lobe 

 becomes narrowed to form the cylindrical proboscis of the adult ; 



and the rudimentary segmentation 

 gradually disappears as development 

 advances. 



In Bonellia there is unequal seg- 

 mentation, as in most Chaetopoda, 

 resulting in the formation of four 

 large"^megameres and eight small 



aphf 



repr.ap 











i/ril 



FIG. 399. Male of Bonellia. all. 

 alimentary-canal ; cael. groups of 

 ccelomic cells destined to give 

 rise to sperms ; repr. ap. repro- 

 ductive aperture ; ves. sem. vesi- 

 cula seminalis. (After Greef.) 



an. 



FIQ. 400. Trochophore of Echiurus, 

 an. anus ; ap. pi. apical plate ; Int. in- > 

 testine ; mo. mouth ; ne. co. rudiment * 

 of nerve-cord ; ces. oesophagus ; ces. 

 conn, cesophageal connective ; stom. 

 stomach. (After Hatschek.) 



micromeres : the latter multiply 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 give rise to a continuous 

 mesoderm layer superficial to the megameres. 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 



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