VII 



ANNELIDA 



143 



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

 situated at the front rim of the long oval blastopore. Cells Sa^ 

 and 5ci also form part of the front wall of the mid-gut, being situated 

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

 the left by 5B' above, by 5a.\ in the middle, and 4a^ below ; and on 

 the right by the corresponding cells 5B', Sc^, and 4rC\ 



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

 formed on the left by SA^', on the right by 50% and in the mid-dorsal 

 line by SDl The valve which projects into the cavity of the gut 



Fig. 106. — Optical sagittal section of young Trochophore after gastrulation is complete. 



(After Woltereck.) 



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, 5Bp and 5Cp, Sc^^ and Sd^^ ; and in addition by 5T>^^ in the 

 mid-dorsal line. 



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

 bc^'- on the right, and Sd^^, 5d}^, and Sd'^^ on the left side, and by 

 the single cell BD^^ in the mid-dorsal line. Its hinder wall is 

 formed by 4d'^, 4d'2, 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 



