ON THE ANATOMY OF HISTRIOP.DELLA TlOMARl. 319 



arms are folded to their full extent the fulcrum is drawn back 

 on a level with the extremity of the blades of the lower jaws. 

 These are paired throughout. They consist of two long wide 

 blades, thickened at their outer margins, and articulating at 

 their basal ends with two curved wedge-like pieces, the 

 pointed end of the wedge being directed forward in the 

 ventral lip of the mouth. Yentrally they articulate with one 

 another in the median line, and turn up dorsally to form a 

 support for the upper jaws. The upper anterior angle of 

 each plate is turned outwards and backwards, some of the 

 fibres of the dorsal longitudinal muscle-bands being inserted 

 into it. The internal interior edges of these plates are finely 

 serrated, and evidently assist the teeth in their action. As 

 far as can be judged from Haswell's figures, the shape of 

 these plates differs slightly in Histriobdella from that of 

 Stratiodrilus. They fold up dorsalh^ to a greater degree. 

 The main portion of the lower jaws are the wide blade-like 

 portions Avhicli project backwards parallel with one another. 

 They are widest behind, and taper slightly in front, where 

 thev articulate with the Avedge-like portions. Connecting 

 the upper with the lower jaws are the pieces that Haswell 

 distinguishes as " bridles." Into the posterior extremities of 

 these are inserted the powerful striated muscles. Apart 

 from their action in binding together the jaw-sets I have not 

 satisfactorily determined their function. They Avould seem 

 to be composed of a single curved piece in Histriobdella, 

 and its chitinous substance is broken up into a number of 

 dark hairs where the muscle is inserted, giving it a furred 

 appearance. The powerful nature of these fibres shows that 

 their action in pulling on the bridles has to do with some 

 essential movement of the jaws. It is likely that the actual 

 process of biting is brought about by their contraction, as 

 Haswell has suggested, while the fulcrum has merely to do 

 with their protrusion and opening. In addition to these 

 there is the peculiar bulbular muscular organ, not unlike the 

 snb-oesophageal muscle pad of Dinophilus. This is attached 

 to the posterior ends of the ventral surfaces of the lower 



