426 



ZOOLOGY 



SECT. 



in the albuminous fluid in the interior of the cocoon, and increases 

 rapidly in size, bursting the enclosing vitelline membrane. As 

 the embryo elongates, the mesoderm bands become divided into 

 segments, and the subsequent history of these is essentially similar 

 to what has been already described in the case of Nereis. The 



ect 



blast op 



m, 



mes 



C 



FIG. 335. Early stages in the development of Lumbricus. ./, lateral view of flattened blastulu : 

 B, ventral view of gastrula with slit-like blastopore ; C, lateral view of later stage ; 1i[n.<im-. 

 blastoccele ; blastop. blastopore ; (.ft. ectoderm ; t/u/. endoderm ; ,n, primary mesoderm cell; 

 mes. nirsoderin bauds ; we/-, cell from which the primitive nerve cord (ne. co.) takes origin ; 

 nph. cells taking part in the formation of the nephridia ; st. stoniodamm. (After Wilson.) 



most essential difference between the two forms consists in the 

 non-occurrence in the Earthworm of any free-swimming Trocho- 

 sphere stage. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Chsetopoda are Annulata with the body made up of distinct 

 , which arc usually numerous and similar throughout 



