254 SUMMARY. 



and various plausible suggestions have been made as to the inter- 

 pretation of the characters of the larvae. 



Lankester 1 has suggested that the larvae are essentially similar 

 to those of Molluscs. He compares the main ciliated ring to the 

 velum, but has ingeniously suggested that it represents not the 

 simple velar ring of most molluscan larvae, but a more extended 

 longitudinal ring, of which the gills of Lamellibranchiata are sup- 

 posed by him to be remnants, and to which the Echinoderm larvae 

 with one continuous ciliated band furnish a parallel. 



The foot he finds in the epistome of the Phylactola?mata, and 

 the disc of Rhabdopleura both situated between the mouth and 

 anus, and therefore in the situation of the molluscan 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 interpretations 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-cesophageal ganglion, which 



1 Lankester. "Remarks on the affinities of Rhabdopleura." Qunrt. J. of Micro. 

 Vol. xiv. 1874. 



