4GO 



ZOOLOGY 



SECT. 



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

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

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

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

 the Chsetopoda. The prse-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 



inieromeres: the latter multiply 



repr.ap rapidly, and grow over the mega- 



, j meres so as eventually to enclose 



the latter in a complete layer of 

 ectoderm, save at one point, 

 where there is a gap, the Uasto- 

 pore. Here the ectoderm bends 



. __ co el 



-ves.sem 



'-*'' ' ' 



. 



FIG. 363. Male of Bonellia. all, 

 alimentary- canal ; cat, groups of 

 coeloniic cells destined to give 

 rise ti> sperms ; repr. ap, repro- 

 ductive aperture; r. x. x, ,,i, vesi- 

 culu seminulis. (After Greef.) 



tail 



FIG. 304. Trochosphere of Echiurus. 

 an. anus; ap. }>l. apial plate; int. in- 

 testine ; mo. mouth ; ne. co. rudiment 

 of nerve cord; as. oesophagus; . 

 conn, ifsophageal connective ; utom. 

 stomach. (After Hatschek.) 



inwards to give rise to a continuous mesoderm layer super- 

 ficial 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 

 stomodceum are developed as an outgrowth, at first solid, from 

 the endoderm. The anus becomes formed still later by in- 

 vagination at the hinder end of the body ; and a pair of epidermal 

 vesicles which appear at its sides form the rudiments of the 



