162 



INVEETEBEATA 



CHAP. 



give rise by budding to strings of cells which will form the ventral 

 nerve-cord. Outside these are two cells termed " nephridioblasts," 

 which give rise to strings of cells which separate into groups as the 

 body of the embryo leech lengthens, and form the nephridia. Outside 



these again are the myoblasts, which 

 similarly give rise to cells which form 

 the external circular muscles, whilst 

 finally the two most external cells 

 give rise to strings of cells which 

 broaden oiit and form the posterior 

 ectoderm. All these strings of cells 

 taken collectively constitute the trunk 

 blastema, it will be thus seen that the 

 development of the trunk blastema is 

 almost identical in character in Grio- 

 drilus and Nephelis. 



A few of the nephridia which are 

 formed at first grow to relatively 

 enormous dimensions, and function as 

 excretory organs during development 

 (Sukatschoff, 1900). These larval 

 nephridia later disappear, and one is 

 involuntarily reminded of the fate which befalls pronephros of 

 Vertebrata (Figs. 121 and 122). 



At first the head blastema is widely separated from this trvmk 

 blastema by a large expanse of bare macromeres : in a typical 

 annelidan embryo this expanse would be taken up by the prototrochal 



Fig. 121. — Hinder view of a well de- 

 veloped larva of Nephelis vulgaris. 

 (After Sukatschoff. ) 



Letters as before. In addition, l.neph, 

 larval nephrida. 



S.O. 



Fig. 122. — Larva of Nephelis vulgaris viewed from the side. (After Snkatschoff.) 



l.np.ph^, the anterior, andi.iiepft^, the posterior larval nephridium ; e.o, external opening of one side 

 of nephridium ; i.e, internal blind end of nephridium ; A, anterior end of larva ; D, dorsal surface of 

 larva ; P, posterior end of larva ; F, Ventral surface of larva. 



cells, which are absent here in accordance with the absence of a free 

 larval stage. The strings of cells forming the ectoderm of the trunk 

 blastema rapidly grow forward, beneath them are the two deeper 

 strings formed by the budding of 4a'' and 4d', the mother cells of the 

 mesoderm which have arisen by the division of 4d. The trunk 



