AND ITS ALLIES. 215 



lastly the terminal joint d, wrinkled in its narrow basal por- 

 tion, and expanded at the end, as already described. 



Despite the apparent four joints, I regard the antenna as 

 consisting of the usual two joints. The basal joint a is con- 

 sidered to be an extension of the skin of the neck, forming a 

 sheath for the antenna proper, the separation between b and c 

 as a false joint to permit of the elbowing. This fold is obscure 

 on the convex side of the elbow, and I have never seen c drawn 

 into b, as we would expect if they were truly distinct segments. 



The neck-segment bearing the antenna has the common 

 dorsal processes on each side of the antenna (see Section Fig. 18). 

 This section is of P. tuberculata, Gosse, and this example, at 

 least, had two knobs on each side of the antenna. 



Head. — When the oral segment is expanded, displaying the 

 corona and mouth, some distinctive features are exhibited, 

 when compared with other Philodinadae. 



Corona. — In dorsal view the discs usually appear rather 

 small, but their apparent size varies greatly with the angle at 

 which they are carried. This arises from the fact that they 

 are farther apart dorsally than ventrally, and if they are in- 

 clined forward only the dorsal edge is seen. If they are less 

 inclined forward, the cilia of the ventral edge are also seen, 

 making the disc seem broader. 



I have never managed to detect central setae on the discs 

 of any of the species of the group ; but considering that ex- 

 tremely delicate setae have been found on the discs of some 

 species of the genus Rotifer, and their usual presence in other 

 Bdelloids, I expect that they will be found present here also. 



Pedicels. — The two convex lines connecting the discs, and 

 meeting at an angle in the median line, are not the boundary 

 lines of the upper lip proper, as in most Bdelloids. They represent 

 the pedicels of the discs, seen on account of the unusual feeding 

 attitude, the upper lip being lower down, nearer the rostrum. 

 This constitutes a further resemblance to the genus Rotifei\ 



Eyes. — Ehrenberg states that the eyes of P. acideata are round, 

 and those of P. macrostyla oblong. I cannot regard this dis- 

 tinction as valid. In both species I have seen both round and 



