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 setse have not been referred to by any writer with whom I 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 Callidinse. They may be 

 always present, but, if shorter than the lamellae, would be difficult 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 setae, such as the whiskers of 

 the cat. 



The tactile setse are somewhat flagelliform, tapering and undulate, but thicker 

 than true flagellae. They vary much in length and thickness, but are always 

 considerably thicker than any other setae 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 setae are the longest I have seen. In the 

 act of extending the rostrum, these species often put out first the four long setae 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 setae have been 

 seen in the above-mentioned and many other Philodinse, in all species of Rotifer where 

 they have been looked for, and in a number of Callidinse. Zacharias (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 I have examined. 



Central setse on discs. — The central seta on the disc of a Philodine, or it may be a 

 pencil of very fine setae, is a familiar structure. The seta usually rises from a papilla, 

 which may be of large size. Central setae were known in several Callidinse, but they 

 were supposed to be absent from most species of this genus, from all species of the 

 genus Rotifer, and from one section of Philodina. 



Recently I found that Rotifer socialis (Callidina socicdis, Kellicott, which I transfer 

 to the genus Rotifer) 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 

 Philodinadae. On R. 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 setae which were in active motion. Two curved lines marked 

 the limits of motion in each direction, as we often see in Vorticella, etc. (fig. 23). 

 R. citrinus has similar setae. 



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. alpium, etc.). 



