vii MOLLUSCA 273 



and other microscopic vegetable or animal organisms. Further they 

 are commonly burrowing in their habits. For both of these reasons 

 they require special arrangements whereby supplies of water can be 

 drawn through the body. Such an arrangement is provided by the 

 ctenidia which have become modified in a characteristic fashion, well 

 seen in an ordinary marine mussel {Mytilus). In the first place the 

 ctenidium has become greatly enlarged and its basis of attachment has 

 been shifted forwards to a point near the front end of the body (Fig. 

 113, C), the ctenidium extending back along the side of the body : in 

 correlation with this alteration in size and position of the ctenidium 

 the deep portion of the mantle-cavity containing it is now paired and 

 lateral in position, being covered in by the large mantle-flap lining the 

 valve of the shell. In the second place the axis of the ctenidium, instead 

 of hanging freely, has become laid up against the roof of the mantle- 

 cavity and completely fused with it, so as to form a longitudinal ridge 

 from which hang down the " respiratory lamellae." In the third place 

 these last-named structures are no longer correctly described as "lamellae," 

 for each one has become lengthened out into a fine filament. From the 

 axis of each ctenidium there hang down into the mantle-cavity two 

 rows, an inner and an outer, of these respiratory filaments, each one 

 bent upon itself into a V-shape. The filaments are richly ciliated and 

 hang down parallel to one another and in close apposition, the ciliary 

 movement being such as to draw a constant stream of water from the 

 mantle-cavity, through the narrow slits between adjacent filaments, 

 into the cavity enclosed by the descending and ascending portions of 

 the filaments, whence it passes out to the exterior at the hind end of 

 the mantle-cavity. In the ordinary mussel the filaments are not merely 

 in close proximity : they are held in position by remarkable arrange- 

 ments known as ciliary brushes. Here and there, facing one another at 

 adjoining positions on two neighbouring filaments, is a patch of stout 

 long stiffish cilia which have to a great extent lost their power of move- 

 ment. When two such patches come in contact the cilia show slight 

 movement which causes the two sets of cilia to become insinuated in 

 amongst one another so that they interlock and are held together like 

 the bristles of two brushes which have been knocked together. The 

 result is that the ctenidium in the undisturbed position has the appearance 

 of two plate-like structures (Fig. in, C), an inner {i.g) and an outer {o.g), 

 formed of the inner and outer row of V's respectively — an appearance 

 which finds expression in the technical name Lamellibranchiata frequently 

 used instead of Pelecypoda. In Mytilus the plate-like arrangement is 

 readily disturbed, the plates falling apart into their constituent filaments, 



T 



