EMBRYOLOGY OF CARDIUM. 233 
three years in attaining its full growth, but is able to propa- 
gate at the end of the first year. 
The development of the cockle (Cardium pygmeum), is 
muchthesame. After passing through a blastula and gastrula 
stage, the embryo becomes ciliated on its upper surface and 
already rotates in the shell. On one side of the oval em- 
bryo is an opening or fissure, on the edges of which arise two 
tubercles which eventually become the two “sails” of the 
velum. The next step is the differentiation of the body 
into head and hind body, z.e., an oral (cephalic) and postoral 
region. Out of the middle of the head grows a single very 
large cilium, the so-called flagellum (Fig. 172 A, fl; v, 
Pig 172.-—The Gerslonmert of the cockle shell (Cardium), A 
v, ciliated crown; ji, flagellum. 2B, Veliger stage, with the shell wereloning v, 
velum ; m, mouth ; 22, liver lobes; ¢, stomach 3; 4, intestine ; mt, mantle ; /, foot; 
mi, muscle ; 7, nervous ganglion.—After Lovén. 
the trochosphere ; 
velum). ‘The shell (B, sh) and mantle (mi; ml, muscle) 
now begin to form. From the inner yolk-mass are developed 
the stomach, the two liver lobes (i) on each side of the 
stomach (¢), and the intestine (2). The mouth (m), which 
is richly ciliated, lies behind the velum, the alimentary canal 
is bent nearly at right angles, and the anus opens behind and 
near the mouth. The velum (Fig. 172 B, v) really consti- 
tutes the upper lip, while a tongue-like projection (B, /) be- 
hind the mouth is the under lip, and is destined to form the 
large unpaired ‘‘foot,” so characteristic of the mollusks. 
The shell arises as a cup-shaped organ in both bivalves and 
univalves, but the hinge and separate valves are indicated 
very early in the Lamellibranchs. At the stage represented 
