450 J. H. ORTON. 



III. MECHANISM CAUSING THE FOOD-CUEEENTS IN 



CEEPIDULA. 



The main food-current is produced by the lashings of rows of cilia 

 on the anterior and posterior faces of the gill-filaments. The fila- 

 ments, it has been noted, stand out in a row from the left side of the 

 mantle (seen in Figs. 1 and 2 below the small arrow), being each 

 supported internally by a pair of chitinous rods (see Fig. 5, C). They 

 are free, and placed a little distance apart so that water can pass 

 between them. When examined separately they are seen to be 

 flattened antero-posteriorly (see Fig. 5), except at the tips, where they 

 are flattened dorso-ventrally, so that at this part they touch the 

 adjoining ones (see Fig. 3). In this way, it may be noted, the fila- 

 ments form a complete roof to the food-groove (see Figs. 1 and 2, B). 

 In transverse section the filaments are seen to have four rows of cilia 

 (see Fig. 5), namely anterior, posterior, dorsal and ventral rows (com- 

 pare Fig. 6). The anterior and posterior rows are formed by far the 

 stronger cilia both in appearance and action. These lash the water 

 from the ventral to . the dorsal face of the gill, and are the chief 

 producers of the main food-current (see Figs. 5 and 6, I.e.). For 

 convenience of reference the anterior and posterior rows may be 

 referred to as the " lateral " rows. The ventral and dorsal rows of cilia 

 lash the water along opposite faces of the gill-filaments towards the 

 tips, i.e. from left to right (see Fig. 6). Both ventral and dorsal rows 

 gather the fine particles and deposit them in the food-groove, but the 

 anterior and posterior rows also assist in this process, as may some- 

 times be seen when examining a living filament under the microscope, 

 or even when examining the living animal with a lens. Therefore, 

 when cilia of the anterior and posterior rows wash food towards the 

 food-groove, the direction of their lashing is changed from a ventro- 

 dorsal to a laevo-dextral one. This is a point of some interest, and 

 apparently the stimulus inducing the change of motion is supplied by 

 the particles merely touching the cilia. 



The way in which the different rows of cilia act may be gathered 

 from a glance at Fig. 6, which is a sketch of the end of a gill-filament. 



If such a piece of a filament be cut off — without the flattened tip — 

 and observed in water it will be seen to swim, when unimpeded by 

 mucus, in the direction indicated by the lowest arrow in the figure. 

 This direction, relative to the long axis of the gill, gives some idea of 

 the relative strength of the lateral rows of cilia as compared with the 

 dorsal and ventral rows, for the direction is, of course, the resultant of 

 the action of the two sets of cilia. Hence it is apparent that the 



