16 



inner branchiae ; or, finally, it may pass through the inter- 

 filamentar gaps in the branchial lamellae 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, PI. I.), the viscero-pedal mass (Ped.l 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 (Ben.) 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 (c/., 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 

 ciliated cells which also form the lining to the pedal 



