66 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



the circulation of the blood in the gills of Lamellibranchia, a problem which I hope 

 to get the opportunity of subjecting to a comparative inquiry. 



With regard to the Chamidae the conditions may be stated as follows. In the 

 freely depending branchiophore there are enclosed a venous afferent and an arterial 

 efferent vessel. From the afferent trunk there emanate perpendicular vessels into 

 the posterior demi-branch, where these vessels run along each seeond principal fila- 

 ment. The interjacent principal filaments of the posterior demi-branch are furnished 

 with vessels communicating with the efferent chief trunk. From the anterior demi- 

 branch there also run perpendicular arteriae to the efferent trunk, these too accom- 

 panying each seeond principal filament. The interjacent principal filaments (or 

 interlamellar septa) of the same demi-branch are occupied by venous vessels emanating 

 from the marginal vein in the upper edge of the reflected lamella. These capillaries 

 are not connected with the axial vein. But, on the other hand, junctions may be 

 established between the latter and the marginal vein in the reflected lamella. Such 

 intermarginal or axio-marginal junctions exist in e. g. Echinochama arcinella and 

 Pseudochama ferruginea, but are not found in P. pusilla and Chama jukesi. 



In the anterior demi-branch the afferent vessels, which start from the margin 

 of the reflected lamella, descend into this lamella, and the efferent ones ascend into 

 the direct or descending lamella. Where the interlamellar septa terminate towards 

 the branchial axis and the principal filaments separate, the vessels give off a short 

 branch which joins the distally situated filament and is obliterated towards its end. 

 Thus the afferent vessels reach the branchial axis with their distal capillary ends, 

 the efferent ones the upper margin of the reflected lamella, but no connection of 

 the ends with the respective main trunks ever takes place. On the other hand, 

 however, a communication may appear between these trunks and the terminations 

 of capillaries of the same system, so that in Chama jukesi the arterial ends of the 

 vessels are connected with the axial arteria branchialis, and in Echinochama and 

 Pseudochama ferruginea, as already mentioned, a junction is established between the 

 marginal vein and the axial one. In all cases these Communications occur in the 

 lovver part of the anterior gill. 



In the posterior demi-branch the connections between equivalent vessels are 

 arranged somewhat differently from those in the anterior, inasmuch as the veins 

 have the composed character of ordinary filamentar vessels and, at the same time, 

 axio-marginal connections, because they run on one side down to the edge of the 

 gill (in Echinochama only in the reflected lamella) on the other directly to the post- 

 erior fmargin. These facts are due to the wide extension backwards of the lamella 

 mentioned and the shortness of the direct one. The arteriae appear simpler inas- 

 much as they accompany the principal filaments without any axio-marginal junc- 

 tions. In the posterior demi-branch we also see much more clearly than in the 

 anterior one that the venous interlamellar septa are higher than the small or ob- 

 solete arterial ones. 



According to the above argument the gills (both demi-branchs) of the Chamidae 

 show, with regard to their vascularisation, these remarkable characteristics: the 



