1876.] 235 [Brooks. 



ogy of the Brachiopods, has shown that, if we consider this group by 

 itself, it must be placed with the Annelids. His investigations also 

 show, with equal clearness, that the Brachiopoda are closely related 

 to the Polyzoa, and we must therefore regard them as united by the 

 " Veliger " to the true Mollusca. If we accept the view that the mol- 

 luscan and vermian stems are thus united, the question, — "Are the 

 Brachiopods Worms or Molluscs?" — will be regarded as nothing 

 but a verbal discussion; for this class forms the connecting link be- 

 tween the two groups, and any sharp line of demarcation does not 

 exist. 



We are now prepared to form a provisional phylogeny of the Mol- 

 lusca, which may be stated as follows : 



The Brachiopods are derived from the Vermes; and from the 

 brachiopod stem, but from something very different from any known 

 Brachiopod, the Polyzoa originated. From the polyzoon stem, but 

 not from any known Polyzoon, we have the Veliger. The true mol- 

 luscs have originated as several offshoots from this veliger stem. Of 

 these the Scaphopods seem to be the least specialized, and in most 

 respects nearest to the original proto-mollusc. The Pteropods are 

 the representatives of another offshoot, to which the Cephalopods 

 also seem to belong. The Gasteropods seem to represent several 

 distinct branches. The Prosobranchiata and perhaps the Heteropods 

 being the descendants of one ; the Opisthobranchs and Pulmonates 

 of another ; and the Chitons of a third. From one of these, or per- 

 haps from the branch now represented by Dentalium, the Lamelli- 

 branchs seem to have been derived at a very early period, and to 

 have diverged considerably from the ancestral form, becoming de- 

 graded in certain respects and at the same time specialized in others. 



In this scheme all reference to the Tunicata is omitted, since it 

 will be conceded by all embryologists that, whatever the affinities of 

 this group may be, they are certainly not with the molluscs. 



I have already referred to one serious objection to the view here 

 advocated ; that is, that it fails to account for the remarkable embry- 

 onic forms of certain Pteropods. Huxley has advocated the view 

 that the Pteropoda and Dentalium have an annelidian ancestry dis- 

 tinct from that of the remaining Mollusca. This view would help us 

 to understand the remarkable larval form of such genera as Pneu- 

 modermon, and at first sight would seem to present a way of escape 

 from our difficulty. It fails to account for the perfect agreement be- 

 tween the veligers of the thecosomatous Pteropods and the Gastero- 



