REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. I27 



the suprabranchial chamber; the four suprabranchial chambers lead posteriorly into 

 the cloaca, which in turn opens to the outside water through the exhalent siphon. The 

 entire gill is subdivided by a series of close-set septa, the interlaniellar junctions (except 

 in Margaritana) which separate the interlamellar space into a series of so-called water 

 tubes. Water in the mantle chamber is driven by the cilia guarding the ostia through 

 the lamellae into the water tubes, whence it passes into the suprabranchial chambers 

 and out through the exhalent siphon. The walls of the gill are traversed by blood 

 vessels and lacunar blood spaces, and the current of water which passes through the 

 gill is a respiratory current. 



The water tubes are lined by an epithelium which is cihated, at least in some species, 

 on the inner faces of the lamellae, while it assumes a characteristic glandular nature on 

 the inner faces of the interlamellar junctions. The lamellae and the interlamellar junc- 

 tions are richly supplied with elastic and smooth muscle fibers, which are especially 

 highly developed in the junctions of the marsupial gills of the female — evidently in 

 adaptation to the great distensibility of which the latter are capable. In fact, the 

 purely respiratory and the marsupial gills exhibit a number of structural differences, 

 most of which were recognized by Peck (op. cit.) and which are undoubtedly to be 

 accounted for on the ground of the difference in function between the two kinds of gills. 

 Peck clearly described and figured the anatomical differentiation between the respiratory 

 and the marsupial gill in Anodonta, and pointed out, among other distinguishing marks, 

 the fact that the interlamellar junctions in the latter are not only thicker and wider 

 and are covered by a peculiar folded epithelium, but that they are set much closer 

 together. It will be well here to quote his description (op. cit., p. 59-60): 



The interlamellar junctions in the outer gill plate (the marsupial gill) are, like the vertical vessels, 

 more numerous than those of the inner plate, occurring at intervals of seven filaments. Tliey are long 

 ridges of dense lacunar tissue, running vertically from base to apex of the gill plate, and have a much 

 greater size, measuring more from one lamella to the other than those of the inner gill plate. In fact, 

 they are capable of very great extension, which takes place when the outer gill plate has its interlamellar 

 space occupied by the glochidian young of Anodon (pi. v, fig. 4). This great depth of tlie interlamellar 

 functions of the outer gill plate is their most remarkable feature, as compared with those of tlie inner 

 plate. It is accompanied by a different disposition of the vertical vascular trunks; for, whilst these 

 in the inner gill plate lie in the interlamelUu" junctions, in the outer gill plate tliey lie in the subfila- 

 mentar mass of concreted tissue at the line of origin of the great ridges which act as interlamellar jimc- 

 tions. In consequence of this arrangement there are two vertical vessels in the outer gill plate to each 

 interlamellar junction, whereas there is only one to each junction in the inner plate. The arrange- 

 ment of these parts in the outer gill plate is no doubt correlated with its function as a brood pouch. * * * 

 The difference just noted between the outer and inner gill plates, due to the frequency of interlamellar 

 junctions and their relation to the vertical vessels, is accompanied by a further difference of form, which 

 is obvious when the sections given in plate v, figures 2 and 3, are compared. In the outer gill plate 

 the two lamellae are parallel to one another and of equal thickness. In the inner gill plate the outer 

 lamella is thicker than the inner, and its surface is thrown into a series of folds. 



He figures very clearly the conditions described in both a non-gravid and a gravid 

 outer gill and also in the purely respiratory inner gill, and it is clear from his descrip- 

 tion that the peculiarities of the outer gill of the female are permanent differentiations 

 and are not merely present during gravidity. We have repeatedly observed the same 



