NBMATHELMINTHES 237 



primitive mesoderm cells [teloblasts] of the Annelida is 

 suggested, especially since in multiplying they extend toward 

 the anterior end, and then constitute two rows of cells lying 

 by the side of the entoderm, resembling the mesodermal 

 bands of the Annelida (Fig. 110 A and G). Their subse- 

 quent development, however, is not the same as in that 

 group, for single cells afterwards separate from them and 

 take np various positions between the intestine and the 

 body-wall, without giving rise to a cojlom homologous to 

 that of the Annelida (comp. p. 268). 



The embryo, which up to this time has possessed an oval 

 form, changes in shape, for it becomes curved toward the 

 ventral side (Fig. 110 D) and more elongated. The shape 

 of the entoderm should be considered in connection with 

 this. At first it forms two layers of cells, between which 

 only a narrow lumen exists (Fig. 110 A and C). The latter 

 soon disappears in the posterior part of the embryo, and the 

 cells now arrange themselves one after the other in a row 

 (Fig. 110 D). The lumen is retained only in the anterior 

 portion; there is formed here a depression of the ectoderm, 

 the fundament of the fore-gut, which unites with the en- 

 toderm (Fig. 110 D and E). The mouth lies in the same 

 place where the last trace of the slit-like blastopore, which 

 closed from behind forwards, was visible. Later a lumen is 

 again formed in the remaining part of the intestine by the 

 splitting of the entoderm (Fig. 110 -E7). The entoderm cells 

 at the posterior end, according to the statements of Goette 

 and Hallez, fuse with the ectoderm to form the anus, with- 

 out any depression of the ectoderm being noticeable, whereas 

 Strubell (No. 10) maintains the existence of such a depres- 

 sion. The central nervous system arises by a thickening of 

 the ectoderm in the region of the mouth (Fig. 110 and Z), 

 n) ; the dorsal and ventral parts of the oesophageal ring are 

 said to sever their connection with the ectoderm earlier than 

 do its lateral parts (Ganin). The ventral longitudinal nerve 

 appears to arise from a paired fundament, a condition which 

 has led to a comparison with the ventral longitudinal nerve 

 cords of the Platyhelminthes. In pursuing this idea, there 

 has also been an in3Unation to refer the dorsal lonjritudinal 



