KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. N:0 3. 69 



lamellibranchia there exist intrafilamentar septa that may mark the double purpose 

 of the filament as a passage of simultaneous afferent and efferent blood streams. 

 These disappear as soon as the gill either becomes synaptorhabdic, i. e. develops 

 organic interfilamentar junctions (ef. Ridewood; of special interest is Avicula ar- 

 gentea, fig. 16, p. 212), or remains eleutherorhabdic but develops a marginal con- 

 tinuous vein in the reflected lamella. In the former case the disappearance may be 

 explained, according to the hypothesis given above, by the formation of organic 

 interfilamentar junctions, in the latter case (e. g. Mytilus) the reduetion of the septa 

 may be due to the fact that the entire demi-branch is to be considered as func- 

 tioning like a single plica of the Chama gill, inasmuch as the one lamella becomes 

 entirely afferent, the other entirely efferent. In this case the blood circulation 

 within the gill becomes very different from that in Chamidae and might exhibit 

 sharp contrasts, so that either the direct lamella is afferent (as in Unio; ef. Lang 

 1900) or the indirect one is. In this we have an explanation to the contradictory, 

 but probably quite correct, accounts of different authors of the direction of the 

 blood streams in different forms. Even such a conclusion may be justified as that 

 an unplicated synaptorhabdic gill is to be deduced from an unplicated eleuthero- 

 rhabdic and a plicated of the former kind from a plicated of the latter, thus in the 

 first case a filibranch, in the second a pseudolamellibranch. There seem to be good 

 reasons for deducing from the latter type the gills of Chamidae, as the formation 

 of principal filaments is probably the first step to their extreme complexity. 



Lastly, we have to state that the gills of Chamidae do not exhibit any funda- 

 mental characteristic suitable for drawing a distinction between the two genera of 

 the family, Chama and Pseudochama. 



2. The vascular system. 



In the preceding chapter we have set forth the leading features of the circula- 

 tion of the blood within the gill. This account will now be completed by a survey 

 of the intervisceral blood circuit within the chief part of the body. The Chamidae 

 exhibit in their arterial blood system a peculiarity not met vvith elsewhere, as far 

 as is known, among the Lamellibranchia. The anterior aorta, as soon as it leaves 

 the heart, divides into one dorsal and one ventral trunk. Such a division is not 

 usual in the Lamellibranchs, but the undivided aorta anterior runs forwards on the 

 dorsal side of the stomach. Only in Chama pellucida does this normal condition 

 seem to prevail. The ventral aorta trunk of the Chamidae penetrates the liver and 

 the genital coeca and arrives at the sides of the stomach just beneath its posterior 

 coecum; here it bifurcates, encircles the duodenum from behind and descends along 

 each of its sides into the foot, where it disappears among the venous lacunae. In 

 this ventral branch we thus recognize an arteria visceralis that has been, contrary 

 to the rule, detached from the post-stomachal instead of the oesophageal part of 

 the common trunk. 



