sh.g 



en. 







Microns 



30 



Figure 333. — Optical section of an early stage of devel- 

 opment of the larva of 0. edulis according to Horst. 

 Reproduced from Pelseneer, 1906. bl. — blastopore; 

 en. — endoderm; ec. — ectoderm; sh.g. — rudimentary shell 

 gland. 



the ventral side of the larva (fig. 1^36, pr.). The 

 digestive system consists of tlie mouth (m.) sur- 

 roimded by ciliated lobes; large stomach (st.); 

 relatively short mtestine (int.); and anus (a.). 

 The thickened central ])art surrounded by the 

 prototroch is considered to be a rudiment of the 

 cephalic ganglion. The larval shell (sh.) covers a 

 considerable part of the body and is formed into 

 right and left oval valves t)f equal size and shape 

 joined at the dorsal side of the larva. At the be- 

 ginning of larval life tlie beating of the cilia of 

 the prototrocli is sporadic and disorganized. 

 Within the ne.xt 15 to 20 minutes the larva first 



me 



Microns 



30 



Figure 334. — Optical section of the gastrula stage of de- 

 velopment of the larva of 0. edulis according to Horst. 

 Reproduced from Pelseneer, IttOG. bl. — blastopore; 

 me. — mesodermic cells; sh. — rudiment of shell. 



rotates around its dorsoventral axis and swinas 

 with the ciliated crown directed forward and up 

 toward the surface of the water. The trochophore 

 stage of C. virginica is short; in the laboratory at 

 22° to 24° C. it lasts no longer than 48 hours and 

 in some instances only 24 hours. 



The ne.Kt stage is Icnown as veliger (from the 

 Latin "veliuii", veil; and "gerere", to carrjO- A 

 detailed account of the structure and development 

 of bivalve veliger was made by Meisenheuner 

 (1901) for Dreissensia polymorpha. MacBride 

 (1914) stated that the development of larvae of 

 Pecten, Teredo, Pholas, Cardium and Ostrea (in- 

 cluding Crassostrea) is virtually identical with that 

 of Drciiisensia. The early larval stages of these 

 forms are so similar that their recognition in 

 ])lankton sam])les cannot be made with confidence 

 until their larval shells have been developed. The 

 structm'e of an early veliger of 0. edulis, described 

 by Yonge (1926, 1960), is similar to that of C. 

 nrginica and C. (jigas. The description given 

 below is based primarily on publications by Yonge 

 (1960) and Erdmann (1935) on 0. edulis. 



The veliger (fig. 337) is a highly complex orga- 

 nism containing several larval organs which dis- 

 appear with the end of free-swimming life. The 

 most conspicuous among the larval structures is 

 the velum, v., which is formed by an outgrowth 

 of the lateral parts of the prototroch area in two 

 semicircular folds or lobes bearing large cilia along 

 their margins. The prototroch thus develops at 



mes 







Microns 



30 



Figure 335. — Optical section of the trochophore larva of 

 O. ediilis according to Horst, 1883. Reproduced from 

 Pelseneer, 1906. c. — cilia; ec. — ectoderm; en. — endo- 

 derm; m. — mouth; mes. — mesodermic cells; sh. — shell; 

 St. — stomach. 



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FISH AND WILDLIFE SERVICE 



