57. 



the tests are cast. In the former, the eggs are tiirovci Tree m 

 the water arid the embryos closely resemble those of Yoldia, but 

 are considerably smaller. In the latter species, the embryos are 

 much larger and are somewhat modified by being carried in a mucous 

 case, which is attached to the posterior end of the shell. 



Outside the group, so far as I have been able to leam. 

 Bonders ia is the only other Mollusk Tdiose embryo is known to be 

 provided witii a similar test. 



Comparing the embryo of Yoldia 7/1 th Pruvot's '^^ ac- 

 count of "i^e embryo of Dondei^ia, we find that the fully formed 

 tests of both are co:nposed of five rows of cells, all of ^Aftiich 

 bear cilia. Counting from the anterior, the cilia on the third 

 row of cells in the test of Bonders ia, and the second, third, 

 and fourth rows of cells in the test of Yoldia, are long aod col- 

 lected into bands which surround the embrjros . The bods^ of the 

 enbrjro of Bonders ia protrudes posteriorly during development. 

 No such protrusion takes place with Yoldia. Each is provided 

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

 cast off. Internal changes in the embryos of Bonders ia not being 

 known, farther comparisons are denied \is. 



The youn.g embryos of Bentalium as described and figured 



by Lacaze-Biathiers (^^ and KowalevskyCSy ^^ear a certain resem- 



