LAMELLIBRANCHIA.] MOLLUSCA 689 doubled on themselves in fact and thus form an additional row of filaments (see fig. 133, B). Consequently, each primi tive filament has a descending and an ascending ramus, and instead of each row forming a simple plate, the plate is double, consisting of a descending and an ascending lamella. As the axis of the ctenidium lies by the side of the body, and is very frequently connate with the body, as so often happens in Gastropods also, we find it convenient to speak of the two plate-like structures formed on each ctenidial axis as the outer and the inner gill-plate ; each of these is Fio. 133. Filaments of the ctenidium of Mytilw edulis (after Holman Peck). A. Part of four filaments seen from the outer face in order to show the ciliated junctions c.j. B. Diagram of the posterior face of a single complete filament with descending ramus and ascending ramus ending in a hook-like process. ep., ep., the ciliated junctions ; il.j., inter-lamellar junction. C. Transverse section of a filament taken so as to cut neither a ciliated junction nor an inter-lamellar junction, f.e., frontal epithelium ; l.f.e ., l.f.e"., the two rows of latero-frontal epithelial cells with long cilia ; ch, chitonous tubular lining of the filament ; lac., blood lacuna traversed by a few processes of connective tissue cells ; b.c., blood-corpuscle. composed of two lamellae, an outer (the reflected) and an adaxial in the case of the outer gill-plate, and an adaxial and an inner (the reflected) in the case of the inner gill-plate. This is the condition seen in Area and Mytilus, the so- called plates dividing upon the slightest touch into their constituent filaments, which are but loosely conjoined by their "ciliated junctions." Complications follow upon this in other forms. Even in Mytilus and Area a con nexion is here and there formed between the ascending and descending rami of a filament by hollow extensible outgrowths called " interlamellar junctions " (ilj in B, fig. 133). Nevertheless the filament is a complete tube formed of chitonous substance and clothed externally by ciliated epithelium, internally by endothelium and lacunar tissue a form of connective tissue as shown in fig. 133, C. Now let us suppose, as happens in the genus Dreissena a genus not far removed from Mytilus that the ciliated inter-filamentar junctions (fig. 136) give place to solid permanent inter-filamentar junctions, so that the filaments are converted, as it were, into a trellis-work. Then let us suppose that the inter-lamellar junctions which we have already noted in Mytilus become very numerous, large, and irregular ; by them the two trellis-works of filaments would be united so as to leave only a sponge-like set of spaces between them. Within the trabeculte of the sponge-work blood circulates, and between the trabeculre the water passes, having entered by the apertures left in the trellis-work formed by the united gill-filaments (fig. 138, A, B). The larger the intra-lamellar spongy Fio. 134. Structure of the ctenidia of Kucula (after Mitsukuri) ; see also fig. 2. A. Section across the axis of a ctenidium with a pair of plates - flattened and shortened filaments attached, i, j, k, g are placed on or near the membrane which attaches the axis of the ctenidium to the side of the body ; a, 6, free extremities of the plates (filaments) ; <?, mid-line of the inferior border ; e, surface of the plate ; t, its upper border ; h, chitonous lining of the plate ; r, dilated blood-space ; v, fibrous tract ; o, upper blood vessel of the axis ; n, lower blood-vessel of the axis ; s, cliitonous framework of the axis ; cp, canal in the same ; A, K, line along which the cross-section C of the plate is taken. B. Animal of a male Nuculd proximo,, Say, as seen when the left valve of the shell and the left half of the mantle-skirt are re moved, a.a., anterior adductor muscle; p.a., posterior adductor muscle; v.m, visceral mass ; /, foot ; g, gill ; I, labial tentacle ; La., filamentous appendage of the labial tentacle ; Ib, hood-like appendage of the labial ten tacle ; m, membrane suspending the gill and attached to the body along the line x, y, z, w ; p, posterior end of the gill (ctenidium). C. Section across one of the gill-plates (A, B, in A) comparable with fig. 133, C. i.a., outer border; d.a., axial border; l.f., latero-frontal epithelium; e, epithelium of general surface ; r, dilated blood-space ; h, chitonous lining (compare A). growth becomes, the more do the original gill-filaments lose the character of blood-holding tubes and tend to become dense elastic rods for the simple purpose of sup porting the spongy growth. This is seen both in the section of Dreissena gill (fig. 136) and in those of Anodon (fig. 137, A, B, C). In the drawing of Dreissena the individual filaments /, /, / are cut across in one lamella at the horizon of an inter-filamentar junction, in the other (lower in the figure) at a point where they are free. The chitonous substance ch is observed to be greatly thickened as compared with what it is in fig. 133, C, tending in fact to obliterate altogether the lumen of the filament. And in Anodon (fig. 137, C) this obliteration is effected. In Anodon, besides being thickened, the skeletal substance of the filament develops a specially dense rod-like body on each side of each filament. Although the structure of the ctenidium is thus highly complicated in Anodon, it is yet more so in some of the Siphonate genera of Lamellibranchs. The filaments take on a secondary grouping, the surface of the lamella being thrown into a series of half-cylindrical ridges, each consisting of ten or twenty filaments ; a filament
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