THE CHzETOGNATHA, OR PRIMITIVE WOLLUSCA. 381 



certainly in error in attributing an enterocoelic origin to 

 them in Palndina vivipara, and his statement has been 

 refuted by Tonniges — and this is practically identical with 

 Avhat happens in Sagitta, for even here the lateral meso- 

 dermal pouches lose their enterocoelic lumen, according to 

 Doncaster, and the permanent coolomic spaces appear as 

 splits at a later stage. The only important point of differ- 

 ence is that the paired lateral coelomic spaces of Sagitta 

 remain separate throughout life, the splanchnic mesoblast 

 giving rise to the dorsal and ventral mesenteries, whereas in 

 the Mollusca they generally become confluent, although in 

 every case indications of their paired origin may be perceived 

 in the fact that all organs derived from them are typically 

 paired. 



The four large cells which lie at the posterior end of meso- 

 blastic pouches in a young Sagitta larva remind us of the 

 first large mesoderm cells which are so conspicuous in the 

 larvas of Chiton, Teredo, and other Mollusca. Both ulti- 

 mately give rise to the genital cells, but in the CliEetognath 

 these cells do not form anything but ova and certain ovarian 

 structures (perhaps the oviduct). In the Mollusca the large 

 cells give rise to other mesodermic structures as well. 



The resemblances between Chcetognath and Molluscan 

 nervous systems have already been described ; it remains 

 but to observe that in their development they are absolutely 

 identical. The dorsal and ventral ganglia appear as inde- 

 pendent local thickenings of the ectoderm in which a 

 considerable proliferation of nuclei takes place. A fact of 

 importance is that the "ventral ganglion '^ of Sagitta, like 

 the visceral ganglia of Mollusca, is paired in its origin, 

 being derived from two cell-bands on the ventro-lateral 

 regions of the body (Doncaster). 



The buccal nervous system was said to be mesodermal by 

 Hertwig, but this statement needs further confirmation. The 

 development of the buccal ganglia from the stomodseal 

 ectoderm has been demonstrated in Paludina, and a similar 

 origin is possible in Sagitta. 



