352 



TUNICATA. 



in the ventral middle line (Figs. 160 C and 161 0, en) these, which 

 are arranged in two parallel rows, retain the character of ordinary 

 entoderm-cells and form a permanent cell-strand connected with the 

 intestine, in which we recognise the vestiges of a caudal section of 

 the alimentary canal (Fig. 162 A, 163 A, en). 



FIG. 164. Median sagittal sections of two stages of development of Distaplia magni- 

 larva (after DAVIDOFF). c, rudiment of the caudal section of the intestine ; ch, 

 rudiment of the chorda ; d, enteric cavity ; ec, ectoderm ; en, entoderm ; n, 

 medullary plate ; up, neuropore ; nr, medullary tube. 



In the caudal region, the separation of the mesoderm and the chorda takes 

 place in a very simple way, the archenteron merely breaking up into the two 

 rudiments. These structures, however, are probably to be derived in a way 

 similar to that described above for the anterior region of the body. We 



