SEA-FISHERIES LABORATORY. 193 
inner branchie ; or, finally, it may pass through the inter- 
filamentar gaps in the branchial lamelle into either dorsal 
or ventral suprabranchial chambers and so into the dorsal 
siphon. 
Two regions of the body can be distinguished (figs. 2 and 
3, Pl. I.), the viscero-pedal mass (Ped.J and Ped.2) and the 
portion of the body lying behind this and in front of the 
posterior adductor. This latter portion contains the peri- 
cardium (Per.) and heart, and the renal organ (Ren.) with. 
the terminal portion of the rectum. The viscero-pedal 
mass contains, besides the muscular foot, the greater part 
of the alimentary canal, the digestive gland, and the 
gonads. It is sharply marked off from the posterior region 
by the differentiation of a sub-epidermal muscular sheath, 
but the same epidermal layer covers both portions of the 
body. | 
The pericardium is situated dorsally, occupying the 
whole dorsal area between the viscero-pedal mass and the 
posterior adductor, beneath it is the renal organ, the 
ventral wall of which forms the roof of the ventral supra- 
branchial chamber. 
The viscero-pedal mass is defined by the continuous 
muscular sheath (cf., fig. 11) extending ventrally from the 
dorsal body-wall. In horizontal! section it is elliptical. 
It consists of a proximal or vertical, and a distal or 
horizontal limb which, both in the contracted and relaxed 
condition, form an angle of about 90° with each other. 
The distal limb is directed forward, it is very much 
flattened laterally and has a deep groove, the pedal groove, 
(By’., fig. 3) running along its ventral edge from near the 
tip to a point beneath the axis of the vertical limb. About 
a third of the length of the distal limb, from the tip pos- 
teriorly, has an epidermis composed of short columnar 
ciated cells which also form the lining to the pedal 
