FEEDING OF CEEPIDULA. 4G9 



the inner than on the outer leaflets, and indeed are apparently often 

 absent from the outer leaflets. These cilia arise from little spurs of 

 tissue and have the same curious rotary motion as those which occur in 

 patches on the gill-filaments of the Filibranchs. The action and dis- 

 tribution of these patches of cilia leave no doubt that they effect ciliary 

 junctions between the leaflets in the same way as those of the Fili- 

 branchs effect junctions between filaments. There are also similar 

 patches of larger cilia immediately below and to the side of the 

 groups of the inter-locking cilia on the tips of each leaflet (see Fig. 18, 

 c.d. below O.c.d. and Led.). Doubtless these have also the function 

 of interlocking the leaflets, and are similar to those occurring on the 

 tips of the gill-filaments of Yoldia (see Kellogg, 9, Fig. 78) and Anomia 

 aculeata. It is therefore evident that the gill of Nucula is undoubtedly 

 less primitive as compared with the gill of other Lamellibranchs than 

 has formerly been thought. A fuller investigation of this gill is being 

 made. 



XL THE BEAEINa OF THE GILL CHARACTEES OF NUCULA 

 ON THE RELATIONSHIPS OF THE PROTOBRANCHIA. 



It is clear from the foregoing description that the gill of Nucula — 

 and indeed those of Yoldia and Solenomya may be included — cannot 

 now be regarded as being so primitive and unique as to justify the 

 classification of the Protobranchia as a group co-equal with the Fili- 

 branchia. Ridewood has shown that the gill of Anomia aculeata con- 

 sists on each side of two rows of filaments whose only difference from 

 the leaflets of Nucula lies, I find, in their being narrow and filamentous 

 instead of broad and lamellate. This obviously constitutes only a minor 

 difference. Indeed, the occurrence of ciliated discs on the gill-lamellae 

 of Nucula might fairly be advanced as evidence of higher specialization 

 — along orthodox Lamellibranch lines — than occurs in the gill of 

 Anomia aculeata, since the filaments of this species of Anomia appear 

 to be without ciliated discs excepting at the tips. Further, the ciliated 

 discs at the tips of the Anomia filaments are matched by similar ones 

 in the gill-leaflets of Nucula, and the action and function of the various 

 rows of cilia is, I find, the same in both animals. Moreover, the frontal 

 cilia on the filaments of Anomia occur on the ventral surface as in 

 Nucula, and those on the outer filaments lash in the same direction as 

 in Nucula (see Fig. 18). Ridewood (13, p. 194) has shown that in this 

 species of Anomia the posterior end of the gill on each side is 

 free and that the inner filaments of each side interlock, and that 

 the outer filaments form a junction with the mantle by means of 

 cilia. All these features are found in the gills of Nucula (see Fig. 18). 



