30 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



with an apical plate, and, in either case, the test is finally cast off. 

 Internal changes in the embryos of Dondersia not being known, further 

 comparisons are denied us. 



The young embryos of Dentalium as described and figured by 

 Lacaze-Duthiers 7 and Kowalevsky 6 bear a certain resemblance to those 

 of Dondersia and Yoldia. This is largely due -to three or more rows 

 of cells, each bearing 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 

 embryos of Patella as figured and described by Patten. 9 In either 

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

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

 to inclose the body, the oesophagus will be pulled out into a long narrow 

 tube ventral to the foot, and the position of the blastopore will corres- 

 pond to its position in Yoldia. Furthermore, the position of the foot 

 and shell-gland will correspond, and the alimentary canal will neces- 

 sarily be bent in the same way. 



In a similar manner, 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 typical larvae of gastropods if we take into account the twisting 

 that accompanies development in these forms. 



Thus we may perhaps trace an homology between the test of 

 Yoldia 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* for Chiton, Sigerfoos 15 for 

 Teredo, and Mrs. Drewf, for Cardium. Wilson 18 has described a similar 

 casting of the cells of the velum for Polygordius. 



It seems quite possible that the embryo of Yoldia represents a 

 rather primitive form, and that the typical molluscan larvte may have 

 been developed from an ancestor approaching it in form and structure. 

 If this is true, the probability is that organs of Yoldia and its rela- 

 tives, which now show high specialization, have been developed from 

 the more primitive rather than from the highly complex. 



* Dr. Heath has kindly allowed me to notice this point, although his observations 

 are not yet published. 



f This work has not been published. The species upon which the observations 

 were made seems to be C. pinnulatum, Con. 



