VII 



ANNELIDA 



143 



cells 4b' and 4b r , which, it will be remembered, were originally 

 situated at the front rim of the long oval blastopore. (Jells Sa 1 

 and So 1 also form part of the front wall of the mid-gut, being situated 

 below 4b' and 4b r . The lateral walls of the mid-gut are formed on 

 the left by 5B 1 above, by 5a 2 , in the middle, and 4a a below ; and on 

 the right by the corresponding cells 5B r , 5c 2 , and 4c a . 



The hinder wall of the mid-gut, where it joins the intestine, is 

 formed on the left by 5A a , on the right by 5C a , and in the mid-dorsal 

 line by 5D' 2 . The valve which projects into the cavity of the gut 



oes ret. 



FIG. 106. Optical sagittal section of young Trocliophore after gastrulation is complete. 



(After Woltereek.) 



Letters as in preceding figures. In addition, V, intestinal valve (this is represented in outline) ; 

 oes.ret, larval retractor muscle of oesophagus. 



and tends to separate the cavity of the mid-gut from that of the 

 intestine, is formed by the following pairs of cells, one member of each 

 pair being situated on the right side and the other on the left, 4cP 

 and 4aP, 5B? and 5C?, 5c 22 and 5d 22 ; and in addition by 5D 12 in the 

 mid-dorsal line. 



The intestinal wall is formed by three pairs of cells, 5c 12 5c n and 

 5c 21 on the right, and 5d 12 , 5d n , and 5d 21 on the left side, and by 

 the single cell 5D 11 in the mid-dorsal line. Its hinder wall is 

 formed by 4d 12 , 4d r ~, which at this period form part of the gut wall 

 but which later are the mother cells of the adult mesoderm. 

 Between and slightly behind them, a little later, the permanent 

 anus arises, and since they formed the hinder rim of the original 



