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MOLLUSCA 



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 



Bate 



FIG. 133. Filaments of the ctenidium of Mytilus 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. /.., frontal epithelium ; i./.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 trabeculse of the 

 sponge-work blood circulates, and between the trabeculae 

 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 



Fro. 134. Structure of the ctenidia of Nucula (after Mitstikuri) ; 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, b, free extremities of the plates (filaments) ; d, 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 ; it, fibrous tract ; o, upper blood- 

 vessel of the axis ; n, lower blood-vessel of the axis ; s, chitonous framework 

 of the axis ; cp, canal in the same ; A, B, line along which the cross-section 

 C of the plate is taken. B. Animal of a male Nmula proxima, 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 ; f, 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 ; /./., 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. 1 37, 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 



