EMBRYOLOOT OF CAMBIUM. 



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 pygmmum), is 

 better known. After passing through a morula 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, i.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. 173 A, fl; v. 



Fie. 173._The development of the cockle shell iCardium). A, the trochoephere ; 

 B, ciliated crown ; Jt, flagellum. B, Veliger stage, with the shell developmg ; v, 

 velum ; m, month ; li, liver lobes ; t, stomach ; i, intestine ; mt, mantle ; /, foot ; 

 ml, muscle ; n, nervous ganglion. — After Loven. 



velum). The shell {B, sh) and mantle {mt ; ml, muscle) 

 now begin to form. From the inner yolk-mass are developed 

 the stomach, the two liver lobes {li) on each side of the 

 stomach {t), and the intestine {i). The mouth (wi), 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. 173 B, v) really consti- 

 tutes the upper lip, while a tongue-like projection {B, f) 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 



