Emhryohgy d:c. 0/ Yol Jia limatula, Say. 273 



indicated. The lettering uill enable the reader to identity 

 the different parts. 



The most striking peculiarities in the development of 

 Yoldia are connected with the formation and disappearance of 

 the test. So far as is known, Dondtrsia is the only other 

 mollusk whose embryo has a locomotor test. A short account 

 of the embryology of Dondtrsia banyulensis, illustrated bv 

 three figures, has been published by Pruvot (7). The fully 

 formed tests of both Dondersia and Yoldia consist of five 

 rows of cells, all of which bear cilia. The cilia on the third 



Figr. o. 



II .. 



pa, _ 



Mo 



Reconstruction of an embryo of twelve and a half days, seen from the 

 left side, with the left shell-valve and mautle-lobe removed. 

 aa, anterior adductor muscle ; c(/, cerebral ganglion ; /, foot ; y, 

 gill : 11, posterior prolongation of the left liver-lobe ; ot, otocj'St ; 

 pa, posterior adductor nsuscle ; pff, pedal ganglion ; ?7, right 

 liver-lobe ; sto, stomach ; iff, visceral ganglion. 



row of cells (counting from the anterior) of Dondersia and 

 the second, third, and fourth rows of Yoldia are long and 

 collected into bands which surround the embryos. I'he body 

 of the embryo of Dondersia protrudes posteriorly during 

 development. No such protrusion takes place with Yoldia, 

 Each is provided with an apical plate and apical cilia, and in 

 either case the test is finally cast off. 



The young larva of Dentalium, as figured and described 

 by Lacaze-Duthiers (4) and Kowalevsky (3), bears a certain 

 resemblance to those of Dondersia and Yoldia. This is 

 largely due to three or more rows of cells, each of whicli 

 bears a band of cilia. At this stage these cells form the 



Ann. d: Mag. N. Hist. Ser. 7. Vol. i. 21 



