142 Affinity of Mollusea and MoUuscoida. By W. K. Brooks. 
bryology 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 " Yeliger " from which the true Mollusea are descended ? 
It is only necessary to glance at the side view of any fully de- 
veloped 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 ganglia, and in Ehabdopleura the epistome is 
functionally as well as morphologically a creeping disk. The shell 
and operculum answer to the cell and lid of a cheilostomatous 
Polyzoon, and the retractor muscles are clearly homologous. The 
most important diff'erences seem to be that, among the Polyzoa, the 
animals are fixed and multiply by budding ; and that in all, the 
mouth, as well as the epistome, is within the circlet of the lopho- 
phore. (Ehabdopleura 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 objection 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 be supposed, however, 
that the Yeliger can be traced back to any existing form of Poly- 
zoon, or even to any order of this class. In some respects its 
affinities are with the Hippocrepia, in others they are with the 
Cheilostomata, and in still others they are with Ehabdopleura, and 
they therefore indicate that the common ancestral type of the Mol- 
lusea was, not a true Polyzoon, but simply a Polyzoon-like form. 
A lack of agreement in points of detail is therefore no more than we 
should anticipate. In answer to the second objection, that the 
Polyzoa multiply by budding, we may refer to the well-known law, 
that agamic vegetative multiplication is antagonistic to high evolu- 
tion, and is accordingly replaced by true sexual reproduction in the 
higher forms of all classes of animals ; as its preseoce, if it occurred 
in any of the true Mollusea, could not be regarded as proof of an 
affinity to the Polyzoa, its absence does not disprove such affinity. 
No one will attach much importance to the remaining objection, 
