168 MR JAMES MURRAY ON 
perhaps giving direction to the weak currents setting towards the discs. Sometimes a 
few of the cilia act alone in a less automatic fashion. 
The straight sete have not been referred to by any writer with whom | am 
acquainted. They (see figs. 2a, 2b, and 3) are generally present in the central group of 
the genus Philodina, and they have been seen in many Callidine. They may be 
always present, but, if shorter than the lamelle, would be dithcult to see. They vary 
greatly in length, are conspicuous in P. rugosa and P. acuticornis (fig. 3), and reach 
the maximum yet observed in P. brevipes (figs. 2a, 2b). Their function may be 
supposed to be the same as that of other motionless rigid sete, such as the whiskers of 
the cat. 
The tactile sete are somewhat flagelliform, tapering and undulate, but thicker 
than true flagelle. They vary much in length and thickness, but are always 
considerably thicker than any other sete on the rostrum. In some species they are 
unmistakably paired ; in others where they are very small it is uncertain whether there 
are one or two on each side of the tip. Their motions sometimes appear automatic, 
but often they seem to be under intelligent control. In P. macrostyla and the related 
species, P. aculeata and P. spinosa, these setee are the longest I have seen. In the 
act of extending the rostrum, these species often put out first the four long setz as 
feelers. They undulate slowly, are separated and brought together again, and the 
animal appears to be feeling if it is safe to come out (fig. 21). These setze have been 
seen in the above-mentioned and many other Philodine, in all species of Rotifer where 
they have been looked for, and in a number of Callidine. ZacHartas (55) figures 
them of great length, on the ventral side of the brush in Rotifer vulgaris; but they 
appear to be dorsal in the species | have examined. 
Central sete on discs.—The central seta on the dise of a Philodine, or it may be a 
pencil of very fine setze, is a familiar structure. The seta usually rises from a papilla, 
which may be of large size. Central setae were known in several Callidine, but they 
were supposed to be absent from most species of this genus, from all species of the 
genus fotifer, and from one section of Philodina. 
Recently I found that Rotifer socialis (Callidina socialis, Ketuicorr, which I transfer 
to the genus Rotter) had in place of the central seta a cluster of short motile cilia 
(fig. 15a). This led me to expect that some modification of the central seta would be 
found in other species of Rotifer, and perhaps throughout the whole of the family 
Philodinadee. On FR. tardus there has been detected a single curved seta, of extreme 
tenuity, apparently motionless (fig. 22). A large variety of R. vulgaris, found in 
Loch Tay, had shorter setee which were in active motion. Two curved lines marked 
the limits of motion in each direction, as we often see in Vorticella, ete. (fig. 23). 
FR. citrinus has similar sete. 
The papilla from which the central seta springs is generally small; it may be 
entirely absent ; or the greater part of the summit of the disc may be produced into a 
conical base for the seta (P. alpiwm, ete.). 
