53. 



blance to those of Bonders ia and Yoldia. Tliis is largely due to 

 three or more rows of colls, each bearing a "band of oilia. At this 

 stage, these ciliated cells form tlie greater portion of the sur- 

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

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

 seem to be cast off. 



A some^rhat similar resemblajice is noticeable in the case 

 of the enbr3ros of Patella as figured an.d described by Patten ^ K 

 In either Dentalium or Patella, if we imagine the velum to be 

 stretched posteriorly over the shell— gland dorsally, and tlie foot 

 ventrally, so as to enclose the bods'", "tlie oesophagus will be pull- 

 ed out into a long narrow tube ventral to the foot, and the posi- 

 tion of the blastopore will corresppnd to its position in Yoldia. 

 Furthermore the position of the foot and sl^ell gland will corres- 

 pond, and the alimentary canal will necessarily 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, tlaere ?^ill be the same relation of pax-ts. The 

 same seems to be true of the larvae of gastropods if we take into 

 account the twisting that accompanies development in these Jforms. 



Thus we may perhaps trace an homology between the test 



