70 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



The dorsal branch of arteria anterior passes above the stomach forward, giving 

 off small laterals to the mantle and to the liver and then descends again along the 

 left side of the oesophagus, in the similar way in Chama and Pseiidochama. In the 

 vicinity of the mouth it gives off a branch towards the front anterior adductor and 

 mantle and then descends into the foot as the arteria pedalis. In one case, a spe- 

 cimen of Chama sarda, it was observed that a small branch emerged from the oe- 

 sophageal portion of the arteria and proceeded backwards to the stomach, here 

 forming, as it seemed, a circular vessel round the duodenum, and similar conditions 

 may prevail in Chama pellucida. In this small collateral there may certainly be 

 seen a homologon to the arteria visceralis of other Lamellibranchia. Whether it is 

 to be considered as a rudimentary or a vestigial feature is, in the present stage of 

 our knowledge, impossible to decide. 



Both Chama and Pseudochama exhibit precisely the same conditions with regard 

 to the arteria visceralis and the position of its oesophageal portion on the left of 

 the oesophagus. There may be some (individual?) variation in the mode of forking 

 and the size of the different branches; thus, for example, Pseudochama jerruginea 

 and cristella have an accessory lateral aorta running on the left side of the body 

 towards the front and then proceeding as the arteria pedalis into the foot, while 

 the dorsal trunk, passing above the stomach, soon disappears above the oesophagus. 

 The sole exception to the rule that the aorta anterior descends on the left of the 

 oesophagus, is presented, according to Grieser, by Chama pellucida, where this 

 vessel was said by this author to run along the right side of the oesophagus, and 

 to give off a collateral to the right cerebral ganglion instead of, as usual, to the 

 left one; for the same species I have, however, found the normal conditions (ef. 

 above). 



The above-mentioned peculiar bifurcation of the arteria anterior immediately 

 in front of the heart has not previously been observed. Ménégaux, who has studied 

 the circulatory system in Pseudochama riippeUi, says nothing about its existence nor 

 does Grieser mention anything about it in Chama pellucida. It seems to me that this 

 fact is, however, of considerable importance and that it may contribute to our under- 

 standing of the relations of the present group. 



Otherwise the arterial system presents~no peculiarities. Towards the posterior 

 portion of the body a posterior aorta emanates from the heart ventrally to the 

 rectum; it soon gives off a ventral branch to the nephridium and to the ganglion 

 viscerale, but is then continued along the under side of the rectum, till it furcates 

 and ramifies into the siphons and the posterior part of the mantle margin. 



The venous system begins, throughout the body, in the intervisceral lacunae; 

 these are most common along the anterior and the under side of the visceral hump. 

 From the foremost portion emanate the anterior veins, which run immediately be- 

 neath the line of f ixation of the anterior demi-branch ; at the point where this passes 

 into the dorsal margin of the reflected lamella the vein plnnges down into the margin 

 and runs along it all round the ventral bend at the end of the gill, where it joins 

 the axial vein. This emerges from the renal sinus round the nephridia, which in 



