517 



Lacaze-Duthiers (4), and by Kowalevsky (3), bear certain resem- 

 blances to the embryos of Dondersia, Yoldia and Nucula. This is 

 largely due to three or more rows of cells, each of which bears a band 

 of cilia. At this stage these ciliated cells form the greater portion of 

 the surface of the embryo. As the body elongates posteriorly, these 

 cells are crowded forward to form the velum, but this does not seem 

 to be cast off. 



A somewhat similar resemblance is noticeable in the case of the 

 embryo of Patella as figured and described by Patten (6). In either 

 Dentalium or Patella, if we imagine the velum to be stretched poste- 

 riorly over the shell-gland dorsally, and the foot ventrally, so as to 

 enclose the body, the oesophagus will be pulled out into a long tube, 

 ventral to the foot. Furthermore the positions of the foot, shell-gland, 

 and alimentary canal will correspond with their position in Yoldia and 

 Nucula. Again, if we imagine the cells of the velum of a typical 

 Lamellibranch larva to be drawn posteriorly over the shell there will 

 be the same relation of parts. The same seems to be true of the 

 larva? of all Gastropods, if we take into account the twisting that 

 accompanies development in these forms. 



We may thus, perhaps, trace an homology between the test of 

 Yoldia or Nucula and the velum of other forms. 



In this connection it is of interest to notice that in a few forms 

 the velum is known to be cast away. This has been observed by 

 Heath x ) for Chiton, by Sigerfoos (9) for Teredo and by Mrs. Drew 2 ) 

 for Cardium. Wilson (11) has described a similar casting of the cells 

 of the velum, for Polygordius. 



It seems quite possible that the embryos of Yoldia and Nucula 

 represent a rather primitive form, and that the typical molluscan 

 larvae have been developed from an ancestor approaching them in 

 form and structure. If this is true, the probability is that organs of 

 members of the Protobranchia, which now show high specialization, 

 have been developed from the more primitive rather than from the 

 more complex. 



Summary. 



A brief statement of the chief points is as follows: 

 Anatomy and Habits. 



1) The mantle of Y. limatula has two pairs of sense organs, an 

 unpaired siphonal tentacle, and a fringe of marginal tentacles. All 



1) Dr. Heath has kindly allowed me to mention this point although 

 his observations are not yet published. 



2) The species upon which this observation was made seems to be 

 C. pinnulatum Cox. The work has not been published. 



