Figure 339. — Photomicrograph of live, slightly narcotized umbo larva of C. virginica. 



, 



umbo larva of this species, slightly narcotized to 

 reduce its movements. 



The anatomy of fully developed oyster larva is 

 known primarily from the work of Erdmann 

 (1935) on 0. edulis. Figure 340, reproduced from 

 his publication, shows the velum (v.) with a crown 

 of powerful ciHa arranged in a preoral ciliated 

 circle, and a ciliated aboral belt or zone covei'ed 

 with small cilia (ab.c). 



Four pairs of velar retractors (r.v.) withdraw 

 the velum. The muscle bands consist of bundles 

 of cross-striated fibers along the dorsal side of the 

 body. The cross striation of the velar retractors 

 of oyster larva is typical for rapidly contracting 

 muscles. In swimming the veliger rapidly changes 

 the degree of expansion and the position of the 

 velmn, and withdraws the organ with great rapid- 

 ity when the valves begin to close. The striated 

 muscles in the larva indicate the high degree of 

 specialization of larval organs needed for the 

 organism to function effectively. The muscles of 

 an adult oyster are nonstriated. Tlieir contrac- 

 tions are relatively slow and do not require tlie 

 mechanism typical for the rapid movements of 

 the free-swimming organism. The apical sense 

 organ (a.p.o.) ("Scheitelorgan", according to 



LARVAL DEVELOPMENT AND METAMORPHOSIS 



Erdmann) and cerebral ganglion occupy a central 

 position in the crown of the velum. The function 

 of the apical organ is not known. 



A new feature in the larval anatomy, not pres- 

 ent at earlier stages, is a well-developed foot (f.) 

 covered with strong cilia. The foot is highly con- 

 tractile and can be withdrawn by its retractor 

 muscle (f.r.). A byssus gland (b.g.) with a small 

 duct opening into the mantle cavity (m.c.) is 

 located at the base of the foot. Both the foot and 

 the gland are typical larval structm-es which dis- 

 appear after performing their function during the 

 attachment. Two muscles, the anterior and pos- 

 terior adductors (ant. ad. and post. ad.), close the 

 valves. The mouth (m.) is surrounded by a cili- 

 ated ridge which develops into the labial palps. 

 The esophagus (e.) leads to the stomach (st.), 

 part of which is covered with the gastric 

 shield (g.sh.). The crystalline style sac (cr.s.) 

 and digestive tubules have greatly increased in 

 size, and the ciliary motion inside them is acceler- 

 ated. The intestinal tract (int.) forms a loop 

 and ends in the rectmn (r.), which has an anal (a.) 

 opening into the mantle cavity (m.c). The rudi- 

 ment of heart and kidney is represented by a 

 gi'oup of cells (h.r.) shown in figm-e 340 above the 



359 



