426 



ZOOLOGY 



SECT. 



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

 rapidly in size, bursting^. th enclosing vitelline membrane. As 

 the emlayS(jjifcmgates. tin- 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 



vet 



srtes 



a si 



rn. 



Tries 



n.e-7* 



FIG. 33.J. Early stages in the development of Immbricus. A, lateral view uf flattened blastula : 

 Ji, ventral view of gastrula with slit-like blastopore ; C, lateral view of later 

 Mast.ocofile : Waxtop. blastopore ; ec<. eetodt n ; //<, primary nn'soderm cell: 



nesoderm bands ; /.,. cell from which the primiti\c aerve cord (e. co.) rakes -n-iu-in : 

 i;lls taking part in the formation of the nephridia ; .^. 



most essential difference between the two forms consists in the 

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

 sphere stage. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Chaetopoda are Annulata with the body made up-of distinct 

 , which are usually numerous and similar throughout 



