post, add. 



Figure 353. — Sagittal section of spat of 0. edulis about 24 hours after attachment, ant. add. — disintegrating anterior 

 adductor muscle; ap. — apical area of the velum; e.g. — cerebro-pleural ganglion; f. — foot; mn. — mantle; m. — mouth; 

 post. add. — posterior adductor muscle; s. — stomach; v. — velum; v.g. — visceral ganglion. After Cole, 1938b. 



of Dreissensia polymorpha and unknown to 

 Stafford showed very clearly that the velum 

 of this bivalve disintegrates and is absorbed, 

 and that the apical area comes to lie outside the 

 esophagus and later is fused with the upper lip 

 of tiie moutii to form the basis of the labial palps. 

 Cole (1938b) sliowed in a series of sections of 

 0. ediilift that as the velum collapses almost 

 immediately after setting, its entire structure 

 is moved upward and forward. Most of it is 

 either cast off or disintegrates, and parts of it 

 probably are swallowed. The apical area or 

 apical plate of the velum becomes detached from 

 siuTounding tissues and sinks to a position dorsal 

 to the esophagus below the surface of the body 

 (fig. 353, ap.) where it fuses with the upper lip 

 of the mouth. Subsequently the thickened upper 

 lip extends laterally to form tlie upper labial 

 palps. The cerebro-pleural gangial (e.g.) can 

 be seen underlying the apical plate. In 4S hours 

 all traces of the velum disappear. 



Reabsorption of the foot begins after the dis- 

 charge of the contents of the byssus gland during 

 attaciiment. Tlie foot gradually shrinks and pro- 

 jects behind the mouth as an irregular mass of 



tissue covered with ciliated cells. Phagocytes 

 invade the interior of the foot and digest the tissue. 

 The disentegration of the foot of 0. edulis is 

 completed in about 3 days. 



The fully developed oyster larva has two 

 adductor muscles. The posterior muscle, dis- 

 covered by Jackson (ISSS, 1890), is not found in 

 the early veliger but appears in the umbo larva. 

 Botli muscles are of approximately equal dimen- 

 sions. Following attachment the anterior muscle 

 degenerates while the posterior moves counter- 

 clockwise in the same direction as the mouth and 

 anus. 



The eyespots of O. edulis break down and dis- 

 appear after the first 24 hours of attached life. 

 Tlu' outlines of the epithelial cup become irregular 

 t)orause it is invaded by phagocytes that ingest 

 tlie pigmented eye cells, thus causing the liberated 

 ])igment to lie in irregular clumps. 



Many pliases of larval-metamorphosis, especially 

 of the ( 'raxxostred group of oysters, are inadequately 

 known and need to be more critically studied. 

 With advances in the technitiue of artificial 

 rearing of oyster larvae this gap in tiie knowledge 

 of oyster biology may soon be filled. 



368 



FISH AND WILDLIFE SERVICE 



