ON A NEW niSTRIOBDELLlD. 30& 



pendent movements ; but tbe strength of their muscles and 

 the toothed character of their anterior edges would seem to 

 indicate that they play something more than a merely passive 

 role. 



In addition to the bundles of fibres which play the part of 

 protractors and retractors, the jaws have three other sets of 

 muscles concerned with their movements. One of these is a 

 pair of large bundles of non-striated fibres, each of which is 

 wrapped round the ventral side of the corresponding lower 

 jaw, the fibres running forwards parallel with tlie latter 

 throughout their length. These two muscles are in close 

 apposition with one another along the mid-ventral line, 

 separated, however, by a thin septum of nucleated material 

 continuous with the lining of the head coelom, of which it 

 appears to be a thickening. They are continuous with the 

 retractor fibres behind. The ventral edge of each is in- 

 folded, and becomes continuous with the ventral edge of the 

 corresponding muscle of the second pair. The latter (figs. 

 4 and 7, str. in.) are a pair of bundles of transversely striated 

 muscular fibres, which are in immediate contact with the 

 lower jaws and enclosed ventrally by the muscles just men- 

 tioned; behind they arise from the main shaft of the lower 

 jaw, towards its posterior end; in front they are inserted 

 into the chain of pieces which I have called the bridle. The 

 third set consjists of a number of non-striated fibres which 

 run forwards parallel with and close to the slender central 

 shaft of the upper jaw (fig. 1 ,j.^). 



The precise mode of action of these various muscles is very 

 difficult to determine with certainty. But there cannot be 

 much doubt that the striated bundles bring about some 

 movement which has to be pei-formed with special rapidity 

 and strength. From their connections, and what I have been 

 able to observe with regard to their mode of action, I am 

 led to conclude that, acting through the bridles on the lateral 

 parts of the upper jaw, their function is to bring about 

 the sharp movements by which these are brought to bear on 

 an external object in the act of biting. 



