POLYZOA. 307 



foot. The peculiar prominence between the mouth and the 

 anus in Pedicellina (vide fig. 130 B) and Loxosoma is probably 

 the same structure. 



Finally he identifies my ciliated disc, which as mentioned 

 above is perhaps equivalent to the cement gland in the adult 

 Loxosoma, as the molluscan shell-gland. Lankester's interpre- 

 tations are very plausible, but at the same time they appear to 

 me to involve considerable difficulties. 



There is absolutely no evidence amongst the Mollusca of the 

 existence of a primitive longitudinal ciliated ring, such as he 

 supposes to have existed, and Lankester is debarred from 

 regarding the ciliated ring of the Polyzoa as equivalent to the 

 simple velar ring of the Mollusca, because his shell-gland lies in 

 the centre and not as it should do on the posterior side of the 

 ciliated ring. 



Another difficulty which I find is the invariable ciliation of 

 Lankester's shell-gland a ciliation which never occurs amongst 

 Mollusca. 



It appears to me that a more satisfactory comparison of the 

 larvae of the Polyzoa with those of the Mollusca is obtained by 

 dropping the view that the ciliated disc is the shell-gland, and 

 by regarding the ciliated ring as equivalent to the velum. This 

 mode of comparison has been adopted by Hatschek. 



The larva ceases however on this view to have any special 

 molluscan characters (except possibly the organ which Lankester 

 has identified as the foot), and only resembles a molluscan larva 

 to the same extent as it does a larva of the Polychaeta. The 

 ciliated disc lies according to this view in the centre of the velar 

 area or prae-oral lobe, and therefore in the situation in which a 

 tuft of cilia is often present in lamellibranchiate and other 

 molluscan larvae, and also in the larvae of most Chaetopoda. It 

 is moreover at this point that the supra-cesophageal ganglion is 

 always formed in the Mollusca and Chaetopoda as a thickening 

 of the epiblast (fig. 134, sg.*), so that the thickening of the 

 epiblast in the ciliated disc of the Polyzoa may perhaps be a 

 rudiment of the supra-oesophageal ganglion, which entirely 

 atrophies in the adult after the attachment has been effected in 

 the region of this disc. 



The comparison between the Polyzoon larva and that of a 



20 2 



