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 larvæ 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 Polychæta. The ciliated disc lies according to this view in the centre of the velar area or præ-oral lobe, and therefore in the situation in which a tuft of cilia is often present in lamellibranchiate and other molluscan larvæ, and also in the larvæ of most Chætopoda. It is moreover at this point that the supra-oesophageal ganglion is always formed in the Mollusca and Chætopoda 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 Chætopod becomes very much strengthened by taking as types Mitraria[127] (fig. 134) and Cyphonautes (fig. 133). The similarity between these two forms is so striking that I am certainly inclined to view the larvæ of the Polyzoa as trochospheres similar to those of Chætopods, Rotifera, etc., _which become fixed in the adult by the extremity of their præ-oral lobe_. [127] The larva of Mitraria is figured with the aboral surface turned upwards, instead of downwards, as in the figure of Cyphonautes. The ciliated band is also diagrammatically put in black for greater distinctness. [FIG. 134. TWO STAGES IN THE DEVELOPMENT OF MITRARIA. (After Metschnikoff.) _m._ mouth; _an._ anus; _sg._ supra-oesophageal ganglion; _br._ and _b._ provisional bristles; _pr.b._ præ-oral ciliated band.]