196 MR JAMES MURRAY ON 



The long spurs, more or less curved towards one another, resemble callipers. The 

 flame-cells are narrow and spindle-shaped. The rostrum is short and broad. The 

 dorso-lateral processes of the neck are conspicuous in the creeping attitude (fig. 6). 

 Length of antenna about equal to half diameter of neck. 



The possession of a " symbiotic " foot, i.e. one ending in a perforate adhesive disc, 

 distinguishes C. circinata from the great majority of Callidinse. Of the small group 

 having a foot of this type, all have a greater number of teeth, except only C. symbiotica 

 and C. armata, and both of these species have short spurs of quite a different shape, so 

 that the species cannot be confused with any known species. 



Callidina plicata, Bryce ( 1 ). 



Described by Bryce in 1892. The most important distinguishing character is the 

 expanded, hood-like, posterior trunk-segment. 



One of the very commonest Bdelloids in Scotland, C. plicata is the centre of a large 

 series of forms, some of which have the processes of the posterior trunk-segment greatly 

 produced, while others have them reduced or entirely absent. The latter would not be 

 recognised as C. plicata, but long study of other characters of the type, and of a series 

 of forms in which the processes were progressively reduced, convinced me that those 

 without the processes were also of this species. 



Description of C. plicata (type) (fig. 14). 



General form narrow, elongate, widest in central trunk. Neck and anterior trunk 

 well extended in usual feeding attitude. Trunk closely plicate, the central dorsal pair 

 running out on to the rump, as pointed out by Bryce ; but the rump is marked off from 

 the central trunk by a very deep transverse furrow, so that the longitudinal folds on 

 the rump might more properly be regarded as distinct from those of the central trunk. 



The transverse furrow is much further forward than the lateral constriction separating 

 rump and central trunk, but I believe both belong to the same line of separation of segments. 



Head. — From the neck the head gradually expands upward, the corona being the 

 widest part. The collar is distinct, but not very prominent. The discs bear central 

 setae, springing from small papillae. 



U pper lip. — Though to some extent variable, the upper lip is one of the best 

 characteristics of the species. There are two convex processes, meeting in the middle 

 line, much as in C. habita (3). Between them is usually a smaller convexity, not 

 belonging to the lip. The processes are connected with the collar by a ridge, as usual. 

 From the outer edge of the processes two fine lines pass downward towards the rostrum, 

 converging towards the middle line, giving the lip the appearance of a keystone set into 

 the head. This form of lip I have seen in no other species. 



Rostrum. — Short and broad, and bearing two lamellae, which in dorsal view seem 

 far separated, as in some symbiotic Callidinse. The restlessness of the animal when not 



