Figure 3: Cleavage in Pinctada maxima (Jameson) (from Wada, 1941). 

 A — zygote; B — with two blastomeres; C — with four blastomeres; pb — polar body; 



pi — polar lobe. 



a sail or velum — ^a disk fringed with cilia that serves as a swimming organ. An 

 apical tuft of sensory cilia is located in the center of the velum. The rest of 

 the larval body is covered with a translucent shell through which the internal 

 organs are visible. 



The description of the internal structure of the veliger is based largely on 

 Meisenheimer's (1901) classical description of the development of Dreissena 

 polymorpha. 



Feeding : The digestive system consists of the ectodermal foregut, ento- 

 dermal midgut, and ectodermal hind gut. Capture of food and its transfer to 

 the oral opening is accomplished by means of the ciliary band of the velum 

 (Figure 9). The velum is thus important in feeding as well as in locomotion. 

 According to Yonge (1926), the food particles fi-om the long cilia of the 

 preoral outer band are passed to the adoral band. The distance between them 

 in the veliger of O. edulis is several microns. The length of the cilia of the 

 adoral band is about 8 |j.m and the width of the band about 20 |im. Food 

 particles are encased in mucus as they move along the adoral band toward the 



