524 DAVID BRYCE OX T\^"0 NEW SPECIES OF PHILODIXA. 



this character in the determination of specimens otherwise of 

 doubtful identity. But while the spurs are thus useful for 

 systematic purposes, the foot as a whole is itself interesting as 

 the highest development found among Bdelloid Rotifera of 

 that portion of the body. Those who are interested in these 

 animals will remember that, by some unexplained error, Hudson 

 and Gosse laid down as one of the characteristics of the order 

 Bdelloida that the toes were three in number. It is now well 

 known that in the genus PhUodiiia the toes seem to be invariably 

 four, whilst in the genus Callidina, as at present constituted, 

 there are species with four toes, others with three, and some 

 without any. 



As I am not aware of the existence of any figures which 

 clearly show the plan on which the four toes are arranged in the 

 typical Philodina foot, I have introduced into the accompanying 

 plate a slightly diagrammatic outline of the hinder portion of 

 the foot of Philodina roseola, drawn from an individual under 

 compression sufficient to force out the foot to the fullest 

 extent. 



It is clear from their descriptions that Hudson and Gosse 

 regarded the foot of a Philodina as including all the segments 

 behind that sudden diminution of the width of the body so con- 

 spicuous in Philodina citrina or P. megalotrocha. Inasmuch, 

 however, as this diminution, apparently marking off the foot from 

 the central body or trunk, is scarcely noticeable in some species, 

 and in others does not occur at the corresponding segment of 

 the bodv, this view of the foot limits does not commend itself as 

 reliable or advantageous for comparison. Later winters have 

 therefore unanimously agreed to regard the foot, throughout all 

 the genera of the Bdelloida, as comprising only those segments 

 which are subsequent to the anus. In the genus Philodina there 

 are usually four of these segments, but in P. roseola and P. 

 eryihropthahna there are five, and in P. commensalis, I believe, six. 

 In the two former species the upj^er part of the foot consists of 

 three segments of nearly equal length. In those other species 

 which have a foot of four segments in all, the first segment is 

 usually much longer than the second. It would seem, therefore, 

 that the extra segment has been evolved by the division of this 

 usually long first segment into two shorter segments. In either 

 case the penultimate segment bears the spurs — two dorsal 



