50 COMMISSION OF CONSERVATION 



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

 ectoderm becomes continuous on to the outer surface of the base of the 

 first gill-filaments. 



Otocysts (ear-sacs) occur right and left, below and behind the pig- 

 ment spots as viewed from the side of the larva (Plate V, fig. 32; Plate 

 VI, fig. 4). In sections, where their position can be determined more 

 accurately, they are found to be placed laterally in the proximal part of 

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

 of the first gill-filaments. Each cyst contains about a dozen small 

 otoconia (ear-motes). 



Nerve Ganglia are to be found in three pairs. The first or supra- 

 oesophageal, cephalic ganglia (Plate VI, figs. 3, 9) form a mass (neural 

 plate) at the centre of the base of the velum, in front of where the oeso- 

 phagus joins the stomach. They are protected in front by what appears 

 in longitudinal sections to be a yellowish-brown, flexible, chitinous layer, 

 from which arise muscle-fibres of the velum. Behind the oesophagus and 

 between the otocysts are the pedal ganglia (Plate VI, fig. 4), connected 

 by a commissural nerve. Large visceral ganglia (Plate VI, figs. 6, 7), 

 connected by commissure, are placed apart, in front of the posterior ad- 

 ductor muscle. 



The Heart (Plate VI, figs. 6, 9) or a vessel containing what appears 

 to be blood cells, is situated above and in front of the posterior adductor 

 muscle and more towards the right (in the right umbo) than the left side 

 — the stomach being most to the left. Two vessels lead towards it from 

 the sides of the base of the foot and abdomen. 



Sections of Larvae — transverse, sagittal, horizontal sections, pre- 

 pared by decalcifj'ing in weak acid to remove the hardness of the shell, 

 embedded in paraffin to facilitate holding and cutting, sectioning with 

 a sliding microtome, staining to differentiate and render more apparent, 

 and mounting in the usual way in Canada balsam on a slide and under a 

 cover-slip — have contributed towards a more accurate understanding of 

 the relative positions, sizes, shapes and structure of the organs. Without 

 these it is scarcely possible to study with any degree of satisfaction such 

 parts as the retractor fibres of the velum, the vacuolated cells of the liver, 

 the three sets of ganglia, the heart and the byssus-gland. In the older 

 and larger living larvse these are obscured by depth, pressure, overlying 

 parts and pigmentation. 



Sagittal sections show especially the large space occupied by the 

 velum in front, the position of the oesophagus and foot below, the stomach 

 and heart above, and the anterior and posterior adductor muscles. 



Transverse sections exhibit the asymmetry of the shell, gills, etc., and 

 the mantle with its thickened edges. Anterior ones show the adductor 

 muscle, the velum, the mouth and the tip of the foot. Median ones 



