



300 



Microns 

 Figure 351. — Pro disso conch of Pycnodonte hyolis (L.). Arrangement as in figure 347. From Ranson, 1960. 



disappear and there is an anatomical reorganiza- 

 tion of the permanent organs. At this time the 

 rehitive size of the organs and their orientation 

 are changed. The extent of topographical changes 

 in the relative position of organs during the transi- 

 tion from larva to spat can be appraised by com- 

 paring the position of some of the larval organs 

 with that in the adult oyster. In figure 352 the 

 principal organs of the early larval (1), fully 

 grown larva (2), and of the juvenile oyster or 

 spat (3) of 0. edulis are shown diagrammatically 

 in three drawings oriented along the dorso- 

 ventral axis. The mouth (m.), nearly ventral 

 in the larvae (1, 2), has shifted counterclockwise 

 (when viewed from the right side of the oyster) 



about half the periphery of the larva and in the 

 spat occupies an area in the anterio-dorsal part 

 near the hinge. The position of the anus (a.) 

 changes in the same direction, from the dorso- 

 posterior part in the larva to dorso-ventral in 

 the adult. The retractor muscles of the velum 

 (r. V.) disappear by the end of the larval period 

 and in the spat and adult are replaced by the 

 radiating and marginal pallial muscles. 



The most conspicuous and rapid changes take 

 place in the velum. Davaine (1853) suggested 

 that in 0. edulis the velum is cast off about the 

 end of the larval period, a conclusion not con- 

 firmed by Ryder (1883) and Stafford (1913). 

 Illustrations by Meisenheimer (1901) of the larva 



DORSAL 



DORSAL 



ant add Q 

 post ad 



d.div 



VENTRAL 



I. 



DORSAL 



post add 



VENTRAL 



Figure 3.52. — Diagram showing the changes in the topographical relation of various organs of 0. edulis during the transi- 

 tion from free-swimming larva (1) to fully developed larva ready to set (2) and juvenile oyster (3). From Erdmann 

 (1935). a. — anus; ant. ad. — anterior adductor; ey. — eye; f. — foot; g. — gills; int. — intestine; l.p. — labial palps; 

 post. ad. — posterior adductor; r.v. — retractors of velum; v. — velum. 



LARVAL DEVELOPMENT AND METAMORPHOSIS 



367 



