64 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



From the branchial axis there proceeds on each side of the body one inner 

 (anterior) and one outer (posterior) demi-branch. The inner one is considerably 

 the larger and covers the sides of the body towards the front as far as the mouth, 

 where it becomes fixed in the extension of the interlabial lateral line. The post- 

 erior demi-branch is much smaller, covering about l jz of the former, and extends back- 

 wards to the posterior adductor. Both demi-branchs have well developed descending 

 and ascending lamellae, as has the smaller posterior one too. in which the ascending 

 lamell a forms a supra-axial or post-axial extension or appendix. In the anterior 

 margin of the anterior demi-branch a marginal furrow appears that is absent in the 

 posterior demi-branch. This feature is common to many other Eulamellibranchia, such 

 as Anodonia, Astarte, Cardita, Cylherea, Cyprina (ef. Rice 1898; Wallengren 1905). 



Both demi-branchs are plicated. In each plica there are in different species, 

 and even in the different regions of the same gill, a somewhat varying number of 

 filaments, up to about 30. At the bottom of the interplicatural furrows one prin- 

 cipal filament occurs, which is only a little broader than the remaining (ordinary) 

 ones. Of the strueture of the single filaments Grieser (1913) gives a careful de- 

 scription, to which I refer the reader for details. 



Within each demi-branch theie appear, on the one hand, organic connections 

 between the ordinary filaments, so that they are all joined to their neighbours by 

 means of tubular bars (synaptorhabdic type of gill), and on the other, similar con- 

 nections between the principal filaments of opposite lamellae, by which connections 

 these are joined. The interfilamentar junctions rim perpendicularly to the filaments; 

 the interlamellar ones run along the principal filaments, thus forming septa within 

 the demi-branch. In Chamidae these septa alternate in height, so that generally each 

 fifth one reaches nearer to the axis of the gill, and of the interjacent ones, which 

 end more remote from it, each second one is produced farther than the adjacent 

 ones. An alternation of the septa, though of a simpler nature, is met with in most 

 of the plicate synaptorhabdic gills; only in Veneridae and some other forms (such 

 as Donax, Psa?nmobia, Semele) are the septa all of a uniform height. To this circum- 

 stance we shall return låter. 



In the gills of the Chamidae there exist other connections besides those already 

 mentioned. Anthony and Grieser have paid attention to the so-called »intraplica- 

 tural» junctions between filaments opposite each other within the same plica, a 

 feature that is also observed by Ridewood in Solenocurtus philippinarum and 

 Thracia papyracea (1. c. p. 254, fig. 42; p. 263, fig. 52). Further there exist in 

 certain forms the intermarginal or axio-marginal bars which connect the upper margin 

 of the reflected lamella directly to the axis of the gill. These formations, which 

 have a special interest in connection with the circulation of the blood and which 

 will be further discussed låter on when dealing with the gill vessels, have not been 

 previously observed either in Chamidae or in other Lamellibranchs. 



With regard to the general morphology of the gills in Chamidae a remarkable 

 variation in their method of fixation to the body is met with in Chama as well as 

 in Pseudochama. In one and the same species the ends of the gills may 1) häng 



