FEEDING OF CREPIDULA. 463 



inner edges of the inner leaflets are apposedand kept interlocked by groups 

 of large cilia (see Fig. 18, I.c.d., p. 468). The outer edges of the outer 

 leaflets and the posterior ends of the gills effect similar ciliary junctions 

 with the mantle by means of groups of large cilia (see Fig. 18, O.c.d.). 

 In this way a complete partition of the mantle cavity is effected, and 

 the ventral surface of this partition is utilized as in the higher Lamel- 

 libranchs for food-collection. Food-particles brought to the gill in the 

 main current are arrested by the gill and washed along the edges of 

 the leaflets from the outer leaflet to the inner (see Fig. 18). At the 

 ventral end of the inner leaflets of both sides the collected food is 

 washed anteriorly towards the mouth. Food-particles collected from 

 the gill in this way appear to be gathered up by the appendages of the 

 palps and transferred to the palps, which pass it along into the mouth. 

 Food-particles may be rejected by the palps in Nucula in the same 

 way as in other Lamellibranchs. Eejected food is pushed off the 

 posterior end of the palps on to the foot, and off the posterior face of 

 the foot on to the mantle. The mantle cilia in Nucula collect particles 

 into anterior and posterior ciliated paths which converge at the middle 

 ventral edge of the mantle just as in the American oyster. The obser- 

 vations on the mode of feeding in Nucula are still being carried on. 

 In the light of the observations already made on ISTucula, and especially 

 of those on the ciliation of the gill (see page 467), it would be worth 

 while to re-examine the mode of feeding in Yoldia. Drew (11, pp. 15 

 and 16) was unable to find out whether Yoldia uses its gills otherwise 

 than for pumping water. If, however, the whole gill be examined 

 alive while feeding it with carmine, the secret would soon be out. It 

 is possible that the gill in this form may not be used for food-collecting, 

 and if not, the condition is more interesting than if it is so used. 

 However, from one of Drew's figures of the gill of Yoldia, although a 

 general view (11, Fig. 20), there is good reason for believing that food- 

 collecting occurs in the same way as is described here for Nucula. 



VIII. AN EXPLANATION OF THE DIEECTION OF 

 EVOLUTION IN LAMELLIBKANCHS. 



The mode of feeding in Lamellibranchs, it will now be seen, neces- 

 sitates the sedentary habits which are exhibited by most members of 

 this group. Moreover, there can be no doubt that adaptation to the 

 mode of feeding has been at least one of the main factors in determining 

 the direction of evolution in Lamellibranchs. From the foregoing 

 account of the gill of Nucula, it is clear that the gill in this form pre- 

 sents an early stage in the adaptation of the original respiratory organ 



