136 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



that the zooids of Dero are much less inclined than those of Cheetogas- 

 ter to remain associated after the development of the bud zone has gone 

 so far as to render them capable of independent existence. 



(c.) For Chaetogaster von Bock ('97, Taf. VI. Figs. 9, 13) figures the 

 connective as passing superficial to the lateral longitudinal muscle bands, 

 and describes the brain as arising by the fusion of the ectodermic con- . 

 tributions from the spaces between the dorsal and lateral (" obere Mus- 

 kelliicken"), and from those between the lateral and ventral muscle 

 bands (" untere Muskelliicken "). These cell masses grow dorsad, over- 

 arching the gut, and fuse with the corresponding masses of the other 

 side to form the brain. According to my observations on Dero, the 

 brain is formed wholly by cells which break through the dorsal longitudi- 

 nal muscle band. When the connective is formed, it lies superficial to 

 a detached lateral portion of the dorsal muscle and returns to the body 

 cavity between this and the lateral muscle (the region at which the cells 

 of the lateral line occur). The regions of ectodermal activity respon- 

 sible for the elements of the brain and connective are thus seen to lie 

 immediately superficial to the lower margins of the dorsal muscle band. 

 From this place there is an upward growth to form the brain, and a 

 ventral growth which marks the course of the connective as it passes 

 toward the ventral cord. The position of the mature connective ex- 

 ternal to certain longitudinal muscle fibres is thus rendered intelligible. 

 In a similar way the region from which nervous cells spring on the 

 ventral side, being superficial to the lateral fibres of the ventral muscle, 

 gives rise to elements, a part of which penetrate the ventral muscle band 

 (cutting off a lateral portion of it) to fuse with the ventral nerve 

 chain, while others pass into the body cavity between the ventral and 

 lateral muscles. The position of the connective in its course from the 

 brain to the ventral ganglia is thus determined, and, as in the dorsal 

 region, is superficial to the lateral portion of the ventral longitudinal 

 muscle. 



While it is entirely possible that the differences in the families studied 

 may account for the apparent differences in the origin of the nervous ele- 

 ments, the course of the connective as figured by von Bock would suggest 

 that the region superficial to the lateral muscle band is the place at 

 which the nervous elements in this case arise, and that the more dorsal 

 ingrowth is chiefly, if not wholly, the source of the brain, — the one 

 between the lateral and ventral bands marking the course of the connect- 

 ive, rather than contributing directly to form the brain. Otherwise, it 

 is very difficult to see why the connective should not follow the line of 



