No. 505] 



THE CANADIAN OYSTER 



39 



umbo is a larger, transverse posterior adductor muscle 

 (Figs. 7, 13, 21, etc.). Retractor fibers converge from 

 the velum to points in the umbos and there are intrinsic 

 muscle fibers in the velum, the foot and the mantle. 



About the center of the animal, as viewed from one 

 side, and anterior to the gills are two conspicuous, black 

 pigment spots (eye specks, Figs. 3, 8, 9, 19) that, in 

 transverse sections of the larvae, are found to be situated 

 right and left on the lateral walls of the body, just in 

 front of where their ectoderm becomes continuous on to 

 the outer surface of the first gill filament. 



Immediately behind and below the pigment spots, but 

 on a deeper level, are right and left otocysts (Figs. 8, 9, 

 1G, 19), each containing about a dozen otoconia. Sections 

 show them to be placed laterally in the proximal part of 

 the foot, close to where its ectoderm passes over on to the 

 inner surfaces of the first gill-filaments. Between the 

 otocysts, and of course behind the oesophagus, are the 

 two connected pedal-ganglia (Fig. 19), and at the center 

 of the base of the velum, in front of where the oesophagus 

 joins the stomach, is the supra-cesophageal ganglionic 

 mass (Figs. 13, 14), protected in front by what appears 

 to be a yellowish-brown, flexible, chitinous layer which 

 gives origin to the muscle-fibers of the velum. 



Transverse, sagittal, and horizontal sections (Figs. 

 13-21) of oyster larva?, prepared in the usual way by 

 decalcifying the shells, staining in alum-cochineal, em- 

 bedding in paraffin, sectioning with a Yung microtome, 

 and mounting on a slide in ('anada l»al>am. have rontrib- 



relative positions of the organs. 



Development naturally begins witli small, simple eggs 

 and proceeds to larger and more complex larvae. By the 

 time I had become oriented with regard to the latter and 

 proved to myself that they can actually metamorphose 

 into oyster spat it was of course too late for that season 



