290 



THE INVERTEBRATA 



arrangement exist especially in the Megascolecidae. Here, in addition 

 to the type already described which is distinguished as a mega- 

 nephridium, there are micronephridia of which enormous numbers may 

 exist in a single segment (2500 in Pheretima). These are small tubes 

 which may or may not open into the coelom by a nephrostome. They 

 may exist in the same segment as a pair of meganephridia. There is 



^end. 



Fig. 200. Embryo of Lumbricus foetidus. After E. B. Wilson. A, Lateral 

 view of an embryo in which the mesoblast is unsegmented. B, Ventral view 

 of the same embryo. C, Longitudinal section of a later embryo a little to one 

 side. D, Transverse section of ventral part of the same embryo along the 

 line XY in C. brn. brain; coe. coelomic cavity of mesoblastic somites; ect. 

 ectoderm; end. endoderm; ent. enteron; M.t. mesoblastic teloblast; npb. 

 nephroblasts ; nrb. neuroblasts ; nep. nephridia ; sep. septa ; std. stomodaeum. 



good evidence for supposing that an originally single meganephridium 

 has been broken up into a multitude of micronephridia. In the 

 development of the earthworm Megascolides the segmental organs 

 first appear as cords of cells like meganephridia. These are thrown 

 into a series of loops and each loop is separated from the rest as a 

 micronephridium. 



