14 



particularly that of nutrients from the digestive system to other parts of the 

 body, occurs through the extensive body cavity. Transport of various sub- 

 stances in the cavities is accomplished by means of flexion and extension of 

 muscles. 



The body cavity of larvae of Crassostrea virginica and C. gigas contains 

 two main types of coelomocytes: 



(1) phagocytes participating in the processes of excretion, and 



(2) coelomocytes with smooth endoplasmic reticulum that do not participate 

 in the process of phagocytosis, but evidently process and transport the 

 soluble nutrients secreted by the cells of the digestive system (Elston,1980). 



Excretion : In the larvae of some bivalves, unlike other mollusks (except 

 pulmonale gastropods), the excretory system is represented by larval 

 protonephridia. These have been described in Dreissena polymorpha. Teredo 

 navalis, Ostrea edulis, and Codakia orbicularis (Hatschek, 1980; Meisencheimer, 

 1901; Waller, 1981; Alatalo et al., 1984). Protonephridia originate from the 

 ectoderm and are arranged on both sides of the body under the epithelium, 

 extending from the esophagus to the base of the foot (Figure 11). A 

 protonephridium is composed of two to three cells. Along the sides of the 

 esophagus in the body cavity lie ciliated cells — one for each protonephridium. 

 These cells are usually conical, often with a vacuole, and bear a tuft of very 

 long cilia lying in the protonephridial canal and extending to its excretory 

 pore. The canal is formed by a second cell. In transverse section, it is possible 

 to see the covering of this intracellular canal, isolated from the cytoplasm of 

 the canal- forming cell (Figure 12). The main mass of cytoplasm and the 

 nucleus are situated in the distal part of the cell; in the proximal part, firmly 

 attached to the ciliated cell, the wall of the canal becomes very thin. A third, 

 sometimes poorly discernible cell, delimits the excretory pore of the 

 protonephridium. The pores of the protonephridia are scarcely visible as they 

 lie deep in the mantle cavity (Figure 13). 



In veligers of other mollusks — Pandora inequivalvis, Cardium edule (Creek, 

 1960; Allen, 1961), and others — ^neither protonephridia nor other excretory 

 organs are seen. Possibly this function is taken up by the coelomocytes scat- 

 tered in the body cavity. Elston (1980) has described the diapedesis — passage 

 of coelomocytes loaded with processed substances from the body cavity through 

 the velum tissue into the mantle cavity. 



Locomotion : In the veliger, swimming consists of a vertical rise followed 

 by a passive sinking. The veliger moves by beating the cilia of the ciliary band 

 along the margin of the velum. The velum in bivalve larvae is usually oval. 

 A remarkable large bilobate velum (Figure 14) is found in transoceanic and 

 deepwater species. Such is the velum of the transoceanic larva Planktomya 



