% 
PLATE V. 
Figure 38. — A similar view of a still older embryo. The 
shell, s , has increased in size, and the digestive tract has two 
openings, the mouth, m, and the anus, an, which are very 
near each other on the ventral surface. 
Figure 39. — The opposite side of a still older embryo, in 
which the body-wall begins to fold under the shell, to form 
the mantle, m. 
Figure 40.— Dorsal view of an embryo at about the same 
stage. 
Figure 41. — Dorsal view of an embryo at the stage shown 
in Figure 38, with its valves extended ; rs, right valve of shell ; 
Is, left valve of shell; an, anus; a, anal papilla; ma, mantle ; 
v, velum ; b, body-cavity ; st, stomach. 
Figure 42. — Yiew of left side of a still older embryo ; i, in- 
testine. Other letters as in Figure 41. 
