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 
prouma 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 
cows bearing circlets of long cilia (Fig. 225). A long ciliated 
zellum, like that of many Lamellibranch larvae, is borne in the 
Fig. 225. 
Surface view of a forty-five hour embryo of Yoldia limatula. a.c, apical cilia ; 6, 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 Dentaliuwm (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- 
