1876.] 233 [Brooks. 



The conclusion to be drawn from our present knowledge of the 

 Mollusca, will appear, from this review, to be that all of them are to 

 be traced back to a free-swimming ancestral form, of which the veli- 

 ger embryo is the representative; this seems to be the only way in 

 which we can account for its appearance in at least certain represen- 

 tatives of so many widely separated groups; and the presence of 

 rudiments of it in such forms as Anodonta and the Pulmonates seems 

 to indicate the same conclusion. We have seen that in many of the 

 cases where it is wanting its absence can be reconciled with this the- 

 ory, even with our present knowledge, and we may therefore hope 

 that a more complete acquaintance with the embryology of the naked 

 Pteropods will show that they are not an exception. 



We come now to the interesting question. : what are the affinities 

 of this " Veliger " from which the true Mollusca are descended? 



It is only necessary to glance at the side view of any fully devel- 

 oped veliger, such as Selenka's figure of Tergipes, in order to notice 

 the resemblance to a Polyzoon, and more careful examination shows 

 that the resemblance holds not only in the general plan but in detail. 

 The velum corresponds to the lophophore in position and structure, 

 and subserves, like this, the function of respiration, and probably 

 that of ingestion as well. The heart is absent in both, and the fluid 

 which fills the body cavity and bathes the digestive organs is kept 

 in motion by the contraction of the various muscles of the body. 

 The digestive organs are similar in form and also in their connections. 

 The epistome with its ganglion answers to the foot and pedal gang- 

 lia, and in Rhabdopleura the epistome is functionally as well as mor- 

 phologically a creeping disc. The shell and operculum answer to 

 the cell and lid of a cheilostomatous Polyzoon, and the retractor 

 muscles are clearly homologous. The most important differences 

 seem to be that among the Polyzoa, the animals are fixed and mul- 

 tiply by budding; and that in all, the mouth, as well as the epistome, 

 is within the circlet of the lophophore. (Rhabdopleura was described 

 by Allman as an exception in this respect : Sars however has shown 

 that although the tentacle-bearing portion comes to an end upon the 

 sides of the foot, the line of cilia is continued entirely around it.) 

 The lack of agreement between the positions occupied by the 

 mouth and foot in the two forms seems to be the most serious ob- 

 jection which can be urged against the view here advocated. In 

 answer to it we can only point out that in Dentalium the mouth is 

 formed within the circlet, although the foot is outside it. It is not to 



