Some sorting devices on the gills of Lamellibranchs are described. These are:

1. Utilization of plicae for sorting in two ways; the one (a) in which the frontal currents are in opposite directions in the grooves and on the crests, and particles intended for consumption are carried in the grooves to the safe dorsal channels, e.g. in Pecten; and the other (b) in which the plical grooves open into the safe channel at the bottom of the deep marginal food groove, while the crests lead to a superficial path along the lobes of the marginal groove, e.g. in Pinna.

2. The deep marginal groove of the demibranchs of certain bivalves is able to open and close, thus accepting or rejecting material carried to it by the frontal cilia of the filaments, e.g. Solecurtus scopula.

3. Certain Lamellibranchs have fan-shaped groups of long cilia on the lobes of the marginal groove of the demibranchs, which prevent the entry of large particles or collections of particles, but allow that of small ones, e.g. Musculus marmoratus.

4. In certain bivalves long, stout frontal cilia are found in addition to the usual short, fine ones. The cirrus-like cilia most probably function in the removal of grains of sand and rock from the gills, e.g. Mactra corallina, Barnea parva.

Small gills and large palps are characteristic of certain deposit feeding bivalves with long free siphons, and the presence of large, cirrus-like cilia on the posterior region of the gills beating obliquely forward, provide additional help to draw the current down the long inhalent siphon.

Certain of the ciliary structures described in this paper are clearly adaptive, correlated with feeding difficulties incidental to the habitat. Fine guarding cilia appear to be frequently correlated with the presence of a certain amount of mud or silt in the soil; they are presumably efficient in dealing with the particles of a muddy soil, but not sufficiently robust when mainly coarse material has to be dealt with. A correlation of cirrus-like cilia with sand dwelling and rock and wood boring habits has been observed.

It will be evident from the observations recorded in this paper that a preliminary sorting of material on the gills before it is passed on to the palps is far from unusual in Lamellibranchs. Further research will most probably add other examples.

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