Watt. — On Boltenia pachydermatina. 341 



the one towards the dorsal, the other towards the ventral 

 edge. 



F. Problematical Organs. — Attached along the dorsal edge 

 of the dorsal limb of the great intestinal loop are struc- 

 tures of which I shall say more in the histological part. 

 There is usually a large number of them, varying in size from 

 blocks resembling in shape and size small gonads down to 

 little structures barely distinguishable, appearing as slight 

 folds of the intestinal wall. They lie, as before stated, along 

 the dorsal edge of the dorsal limb of the intestine, always ex- 

 tending on the posterior side to the ventral end of the last bend 

 of the intestine. On the anterior end they may reach to the 

 middle of the first bend of the intestine, but in this case 

 when they reach this pomt they are visible only as very slight 

 folds. As a general rule, however, they extend only along 

 the dorsal limb. They are present in all specimens, but the 

 greatest amount of variation exists with regard to their size 

 and arrangement in different specimens. In some they are 

 of fairly even size throughout, tapering away towards the 

 anterior end to slight folds. In others they are largest at the 

 posterior end, becoming smaller towards the anterior, though 

 they may swell out in size towards the middle of their course. 

 In others, again, they are largest about the middle of the dorsal 

 limb, diminishing in size in front and behind. 



D. Vascular System. 

 The heart lies on the right side, attached to the inner 

 surface of the mantle, in close relation with and parallel to the 

 endostyle. It is long and tubular. From its anterior end goes 

 off a vessel which we may call the ventral vessel. This runs 

 along the right side, and is, like the heart, connected by trans- 

 verse vessels (paired) with the branchial sac. When it reaches 

 the place where the endostyle makes its bend dorsalwards it 

 breaks up into three chief vessels — one to the test, one running 

 back along the endostyle, another running forwards. The 

 vessel running forwards gives off' branches to the anterior pro- 

 longation, and when it reaches the oral end of the endostyle it 

 divides into two vessels running round the circlet of tentacles. 

 The vessel from the posterior end of the heart (dorsal vessel) 

 runs to the posterior end, bends dorsalwards, and when it 

 reaches the point where the gullet passes into the stomach it 

 divides into two vessels, one passing along the ventral surface 

 of the stomach, the other passing round the gullet on to the 

 dorsal surface of the stomach, crossing diagonally to the 

 ventral surface, and giving off in its course branches to the 

 different stems of the digestive gland. This vessel reaches 

 the ventral surface about half-way between the oral and atrial 

 apertures, and, after giving off a large vessel running along the 



