THE LAMELLIBRANCHIA 249 



and the larva is free-swimming. But in Nucula delphinodonta the 

 female constructs a thin-walled mucoid egg-case, attached to the 

 posterior portion of the shell and in communication with the 

 pallial chamber : into this case the ova are passed as soon as they 

 are laid, and undergo their development. In Yoldia and Nucula 

 proxiina a gastrula is formed by epiboly and then the greater 

 part of the ectoderm gives rise to a " test," which is really a 

 ciliated velum formed in a normal position at the apical pole, 

 but reflected in such a manner as to completely cover the former 

 ectodermic surface of the body viz. the shell-gland, etc. leaving 

 only a small opening opposite to the apical plate, in which the 

 stomodaeum and eventually the proctodaeum are formed (Fig. 16). 

 The test consists of five rows of flattened cells, the three median 

 rows bearing circlets of long cilia (Fig. 225). A long ciliated 

 flagellum, like that of many Lamellibranch larvae, is borne in the 



Fio. 225. 



Surface view of a forty-five hour embryo of Yoldia limatula. a.c, apical cilia ; U, blastopore ; 

 x, depression where the cells that form the cerebral ganglia come to the surface. (After Drew. 



centre of the apical plate. When the larval development is com- 

 pleted, the test, with its stalk and apical plate, is stripped off and 

 cast away within the space of a few minutes (Fig. 226) ; the apical 

 cilia shrivelling up and the test cells breaking apart and frequently 

 falling to pieces at once. In the larva of Nucula delphinodonta the 

 test is covered with short diffuse cilia, there is no flagellum, and 

 the disruption and casting off of the test occupies several hours, 

 the parts near the apical plate being the last to disappear. The 

 testaceous larvae of the Nuculidae should be compared with the 

 larvae of Dentalium (Fig. 15) and of Myzomenia (Fig. 17). 



A development with secondary metamorphosis, acquired in the 

 course of ontogeny, is peculiar to the Unionidae. In this family 

 the eggs are laid in spring or summer, and on leaving the genital 

 orifice pass into the interlamellar space of the internal gill-plate ; 

 thence into the interlamellar space of the outer gill-plate by way 

 of the posterior extremity of the gill, where the two spaces com- 



