338 GII;MAN A. DREW. 



process of casting occupies several and sometimes as many 

 as fifteen hours. The process is much more rapid for both 

 Yoldia limatula and Nucula proxima (2), It is quite 

 possible that the difference in the length of the time occu- 

 pied by the different embryos is connected with the differ- 

 ence in the conditions under which they develop. 



Further study has tended to confirm my view that the test 

 should be regarded as the homologue of the velum of other 

 forms. In a former publication (1) I made the statement 

 that " in either Dentalium or Patella, if we imagine the 

 velum to be stretched posteriorly over the shell-gland dor- 

 sally, and the foot ventrally, so as to enclose the body, the 

 oesophagus will be pulled out into a narrow tube ventral to 

 the foot, and the position of the blastopore will correspond 

 to its position in Yoldia. Furthermoi-e the position of the 

 foot and shell-gland will correspond, and the alimentary 

 canal will be bent in the same way." This states the case 

 backward, and may be a little confusing. If we begin with 

 the condition found in Yoldia and Nucula, and imagine the 

 test to shrink until it consists of a band of ciliated cells sur- 

 rounding the embryo anterior to the mouth, the condition 

 would be comparable to that shown by embryos of Den- 

 talium and Patella. 



As in the case of Yoldia, the closest resemblance to the 

 test, outside of the group, is shown by Dondersia. Although 

 Pruvot's (15) account of these embryos is very short, and 

 only three figures are given, there is quite a striking external 

 resemblance. In both cases the surface cells are arranged 

 in five rows, all of which bear cilia. They are both provided 

 with apical plates, and with both the test is finally thrown 

 away. The bodies of the embryos of Dondersia protrude 

 posteriorly during development. A slight posterior protru- 

 sion of the body of Nucula sometimes takes place through 

 the opening dorsal to the blastopore. 



The resemblances shown by embryos of Dentalium 

 (8 and 9) and Patella (12) are not so striking, but they are. 

 somewhat similar. The apparent posterior protrusion of the 



