EMBRYOLOGY OF CARDIUM. 



2-3J 



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 {Cardiiim vygmmum), 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 oyal em- 

 bryo is an opening or fissure, on the edges of which arise two 

 tubercles which eventually become the two "sails" of the 

 Telum. 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. 172 A, fl; v. 



Pig. 172.— The development of the cockle shell iCardium). A, the trochosphere ; 

 tJ, ciliated crown ; Ji, flagellum, B. Veliger sta£;e, with the shell developmg ; v^ 

 velum ; m, mouth ; ti, liver lohes ; t. stomach ; J, intestine ; mt^ mantle ; /, loot ; 

 nU, muscle ; ft, nervous ganglion, — After Lov^n. 



velum). The shell {B, s7i) 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 (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. 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 

 "^"ery early in the Lamellibranchs. At the stage represented 



