42 . J. H. ORTON. 



compared to that of the bristles of two brushes when the brushes are 

 rubbed together slowly in a rotary manner with the bristles interlocking. 

 To get the proper effect one brush should be started rotating before the 

 other, and the bristles should be regarded as vibrating at the same time 

 with a slow wave-like motion which begins at the attached end. 



The function of these ciliated knobs is doubtless the same as that of the 

 ciliated discs of the Filibranchia, namely, to interlock with similar discs 

 on the leaflets of the gill on the other side of the body ; or, in the case of 

 the anterior leaflets, with cilia on the side of the body of the animal. 

 When suitable material can be obtained sections of the whole gill will be 

 made to show the interlocking of these cilia. The number of ciliated 

 knobs on the gill-leafiets varies in different parts of the gill. Thus in one 

 specimen examined there were only two or three on the anterior upper 

 leaflets and twelve in the corresponding lower leaflets. Behind this point 

 the knobs increase in number, so that at the beginning of about the 

 middle third of the gill there were ten or eleven in the upper and thirty 

 to forty in the lower. In about the middle of the gill there are still more 

 knobs on both leaflets, thirty-five to forty being counted on the upper 

 ones, but the number on the corresponding leaflet was not made out. 

 In front of the posterior end about thirty-six knobs were counted in the 

 upper and eighteen to twenty in the lower leaflets. There are thus more 

 knobs in the middle of the gill and fewer at the anterior and posterior ends, 

 and more in the lower leaflets in the anterior end than in the upper ones 

 in this position, but more in the upper than in the lower at the posterior 

 end. These differences are doubtless explained by the facts that at the 

 anterior ends of the gills the upper leaflets appear to lie against the body, 

 while the lower ones meet below, and thus require a better interlocking 

 arrangement, while at the posterior end of the gill the upj)er leaflets are 

 longer than the lower ones. 



Besides these ciliated knobs there are also other interlocking cilia 

 at the tips of the upper and at the ventro-lateral edges of the lower 

 leaflets (see Fig. 10 I.e.). These cilia doubtless effect a junction with the 

 mantle, and an attempt will be made to obtain sections of the gills and 

 mantle to show these connexions. 



On the anterior and posterior faces of the ventral edges of the lower 

 leaflets there are patches of cilia which also have the rotary motion 

 characteristic of interlocking cilia. These probably serve — like the 

 similar patches on the gill leaflets of Nucula (see Fig. 11, c.d.) — ^to hold the 

 tips of the leaflets together. Thus the living gill of Solenomya is a very 

 compact organ, which, however, like that of Pecten, can be contracted 



