464 J. H. ORTON. 



to a food-collecting organ; and further, recent work (see Pelseneer, 12, 

 pp. 253-4, and Ridewood, 13) has confirmed the conclusion that the 

 higher Lamellibranchs have evolved mainly on the principle of folding 

 and consolidating the originally simple free gill-filaments to form gill- 

 lamellae. It will now be seen to be highly probable that this com- 

 plication and fusion of the gill-filaments is an adaptation for the 

 purpose of obtaining a more efficient feeding organ. Folding of the gill- 

 filaments dorso-ventrally into demibranchs and — incipiently — antero- 

 posteriorly into crests and troughs has increased the food-collecting 

 surface, while fusion of the filaments first by ciliary jvmctions and 

 afterwards by organic connections has rendered the food-collecting 

 organ less liable to derangement. In the Filibranchs there is much 

 danger of the gill-filaments becoming separated, whereby the con- 

 tinuity of the food-grooves at their tips is broken. As a result the 

 animal may have difficulty in feeding, and its nourishment be thus 

 seriously interfered with. Hence adaptations which ensure a firm gill 

 would undoubtedly be advantageous — other things being the same — 

 in preventing interference with the feeding process. The folding of 

 the gill in an antero-posterior direction is also an adaptation in per- 

 fecting the feeding process, for by this means the food-collecting sur- 

 face of the gill is further increased (see various figures by Ridewood, 

 13, pp. 242-263) ; moreover, greater opportunity is thereby given 

 for effectively sieving the food-current, which has necessarily to pass 

 more obliquely over the surface of the filaments to pass onwards into 

 the exhalent current, thus giving the frontal cilia of the gill-fila- 

 ments a better chance of capturing food-particles. It will also 

 be seen that this folding results in the formation of secondary food- 

 channels, thus the principal and apical filaments which occur in the 

 troughs and crests respectively of the folds of the gills of many 

 Lamellibranchs (see Ridewood, 13, p. 163) probably function mainly 

 as the bearers of subsidiary food-grooves. 



Along with the evolution of a more efficient food-collecting gill in 

 Lamellibranchs there have occurred a gradual fusion of the ventral 

 edges of the mantle lobes and a development of inhalant and exhalent 

 siphons. It is highly probable that this fusion of the mantle lobes 

 is primarily an adaptation of the same nature as the gill folding, that 

 is, tending towards perfecting the mode of feeding. For in Mytilus, 

 Glycimeris, and Ostrea, and doubtless also in many other forms, there 

 is an attempt to limit the ingoing current to a definite area, and the 

 effect obtained is that of limiting the area over which the heavier 

 particles settle out of the food-stream to a part of the mantle adapted 

 for expelling the undesirable material. In siphonate forms with the 



