Brachiopods have been at various times placed with the Mollusca, the Chaetopoda, the Chaetognatha, the Phoronidea, the Polyzoa, the Hemichordata, and the Urochordata.
The histology of the nervous system of Mollusca has yet to be seriously inquired into.
In other Pectinibranchia (and such variations are representative for all Mollusca, and not characteristic only of Pectinibranchia) we find that there is a very unequal division of the egg-cell at the commencement of embryonic development, as in Nassa.
This is the largest group of Mollusca, including nearly sixty families, some of which are insufficiently known from the anatomical point of view.
The Pteropoda, formerly regarded as a distinct class of the Mollusca, were interpreted by E.