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 setae is developed at the anal end, and subsequently 

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

 the Chaetopoda. The prae-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- 



.,-v.^s. meres so as eventually to enclose 



the latter in a complete layer of 

 ectoderm, save at one point, 

 where there is a gap, the llasto- 

 pore. Here the ectoderm bends 



coel 



-ves.sem 

 all 



J. C(HI 



stem 



FIG. 3(53. Male of Bonellia. all, 

 alimentary-canal; coel, groups of 

 coelomic cells destined to give 

 rise to sperms ; repr. ap, repro- 

 ductive aperture ; res. sem, vesi- 

 cuLi seminalis. (After Greef.) 



Vic.. 304. Trochospherc of Echiurus. 

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

 testine ; mo. mouth ; ne. co. rudiment 

 of nerve cord; &s. oesophagus; ot, 

 <},>, i. rasophageal connective; .<fo,,t. 

 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 

 stotnodceum 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 



