516 



mantle, Fig. 19 g. These thickenings grow anteriorly so as to pro- 

 ject over the surfaces of the mantles and acquire strong cilia. A 

 constriction appears near the middle of each, and the two ends 



grow to form lobes, 



__ - aa 



Fig. 21 g. More lobes 

 are added by the 

 growth of the ven- 

 tral lobe. The later 

 development of the 

 gills of N. delphin- 

 odonta, shows that 



the inner plates 

 attain considerable 

 size before the outer 

 plates are formed. 



Fig. 21. Reconstruction of a 10 day embryo of Yoldia limatula. Represented as seen 

 from the right side with the right shell-valve and mantle-lobe removed aa anterior adductor 

 muscle, eg cerebral ganglion. / foot, g gill, int intestine. II left lube of the digestive 

 gland, ot otocyst. pa posterior adductor muscle, pg pedal ganglion, rl right lobe of 

 the digestive gland, sto stomach, vg visceral ganglion. 



While these changes have been taking place, the alimentary canal 

 has increased considerably in size and length. The labial palps are 

 of late formation, and appear as outgrowths of the body wall. 



General Remarks. 



The great diversity in the structure of members of this group is 

 of much iuterest, and possibly points to an ancestor with a very gene- 

 ralized structure. The most striking peculiarities in the development 

 are connected with the formation and disappearance of the tests. 

 Outside of the group, so far as I have been able to learn, Dondersia is 

 the only other mollusk whose embryos are known to be provided with 

 similar tests. Comparing Pruvot's (8) account of the embryo of Don- 

 dersia with the embryos of this group, we find that the fully formed 

 test of each is composed of five rows of cells, all of which bear cilia. 

 On the tests of Dondersia, Yoldia, and Nucula proxiraa the cilia on 

 some of these rows of test-cells are long and collected into bands. 

 All are provided with apical plates, and with all the tests are finally 

 cast off. The bodies of the embryos of Dondersia protrude posteriorly 

 during development. No such protrusion takes place in the embryos 

 here described. As the internal changes in the embryos of Dondersia 

 are not known, further comparisons are denied us. 



The young embryos of Dentalium, as described and figured by 



