340 Transactions. — Zoology. 



branchial sac to the ventral, and carry the blood to the vessel 

 running along the dorsal edge. On the folds we can see that 

 the meshes are narrower than in the interspaces between the 

 folds. This is due to the fact that the internal longitudinal 

 vessels are always more closely placed on the folds than on the 

 interspaces, becoming more closely approximated the nearer 

 we get to the crest of the fold. 



D. Gullet, Stomach, and Intestine. — The branchial sac opens 

 into the gullet, which is attached to the dorsal body- wall at 

 the posterior end of the body. The oesophageal aperture is 

 overhung by a large valvular projection of the left inferior 

 wall of the gullet. The gullet passes down the left side of the 

 body, attached to the body-wall, makes a bend forwards when 

 nearer the ventral than the dorsal edge, and at this point 

 passes into the tubular stomach. This, after running forward 

 for rather less than one-third of the length of the body, 

 passes into the intestine, the diameter of which is very little 

 less than that of the stomach itself. This, like gullet and 

 stomach, is firmly attached to the body-wall. It runs straight 

 along the left side to the anterior end of the body, at the 

 anterior end of the body makes a bend dorsalwards, turns 

 backwards, and runs parallel with its former course. When it 

 comes beneath the atrial aperture it turns dorsalwards at 

 right-angles to its course, and, after running for a short dis- 

 tance, opens into the atrial siphon. All along the attached 

 side of the stomach and intestine is a large typhlosole, which 

 is continued right to the anus. The posterior end of the intes- 

 tine on its attached side projects very little from its attachment 

 to the body- wall ; but on its free side the end of the intestine 

 projects for a considerable distance, overhanging the attached 

 side like a great flap. 



These general relationships are shown clearly in Plate 

 XXXIV., figs. 1 and 2, where the body-wall of the right side 

 and entire branchial sac are removed. 



E. Digestive Gland. — Attached along the right side of the 

 stomach are a number of little tree-like structures. Each 

 consists of a stem giving off branches, which branch again and 

 again, the distal ends of the twigs bearing little tufts of coeca. 

 In living specimens the colour of the digestive gland is bright- 

 orange ; this colour in spirit changes first to dull-red, then to 

 a yellowish-red, and at last gets bleached out to a yellowish- 

 white. There is no great variation in the number of the 

 stems, there being from 13 to 17 present. The arrangement 

 differs somewhat in different specimens, but there is always a 

 smallish stem towards the ventral edge of the right side at 

 the beginning of the stomach, and another stem (the largest 

 of all) at the end of the stomach towards the dorsal edge. 

 Between these the other stems are arranged in a double row. 



