ON CILIARY MECHANISMS. 293 



of the lophophore. The secretions of these latter cells assist in a similar 

 way to that from the filaments in the capture and transportation of 

 food-particles. 



THE CILIATION OF THE GILL-FILAMENTS OF BRACHIOPODS- 



The cilia on the gill-filaments of Brachiopods are arranged in three 

 rows along the length of the filament, as has already been described. 

 There are, however, as Oehlert has observed, two kinds of filaments, a 

 larger and a smaller, as is shown in Fig. 7, p. 292. The smaller ones 

 constitute the ventral series (that is, the series facing the incoming 

 current) on the lophophore and alternate with the larger ones in the 

 dorsal series (see Fig. 5, p. 289). These two kinds also differ in the arrange- 

 ment of the rows of cilia. On the smaller ones the lateral cilia are situated 

 on the sides of the filaments and near the ab-frontal face, while in the 

 larger filaments they occur similarly on the sides, but near the frontal 

 face, except towards the tips of the filaments, where these current-produc- 

 ing cilia graduate towards the ab-frontal face (see Fig. 3, p. 287). The 

 disposition of the rows of cilia is well shown in Fig. 7, p. 292, which 

 represents transverse sections of the two kinds of filaments taken near 

 their origin from the lophophore. In the smaller sections the filaments 

 are shown to be nearly cylindrical and the greater part of the convex 

 face occupied by the frontal, that is, food-collecting cilia. The larger 

 cilia on the sides of the filaments are the lateral or current-producing 

 cilia. Both these rows of cilia arise from columnar cells which have long 

 elliptical nuclei. On the contrary the non-ciliated ab-frontal epithelium 

 is composed of squarish to flattened cells with more or less spherical 

 nuclei, and indeed it would appear that the character of an epithelium 

 on any gill-filament forms a very good guide as to whether or not that 

 epithelium is ciliated ; if the cells in a portion of an epithelium are 

 columnar and the nuclei elongated they probably bear cilia, whereas 

 if the cells are squarish to squamous they are probably not ciliated. 

 Such an interpretation of portions of an epithelium on gill-filaments 

 may be useful in cases where it is difficult to obtain well-preserved material, 

 for as is well known it is difficult to determine what part of an epithelium 

 is ciliated unless such material can be obtained. One good illustration 

 of this fact is afforded by the well-drawn figure of a transverse section of 

 a gill-filament of Rhynchonella given by Van Bemmelen (6, Plate 8, 

 Fig. 6). In this figure no cilia are shown at all, although the characters 

 of the epithelium are seen to be almost identical with that of the smaller 

 sections in Fig. 7, p. 292. A somewhat diagrammatic transverse section 



