NOTES ON THE HISTRIOBDELLID.E. 
209 
the dorsal wall of the liead, embracing between them the 
oesophagus and the dorsal cephalic gland (fig. 13). In the 
living animal, when the fibres of these muscles contract, 
there are synchronous movements of the margins of the 
mouth. 
The second pair are the striated muscles (figs. 2 and 
13, 5. m.), the fibres of which arise from the shaft of the lower 
jaw towards its posterior end and run straight forwards to be 
inserted into the corresponding bridle — a curved chitinous 
piece that slides backwards and forwards along the shaft and 
probably serves as the medium through which the contrac- 
tions of the striated tissue are transmitted to the rami of the 
upper jaw (see 6, p. 309). 
A third set of muscular fibres arise from the fulcrum of the 
upper jaw, and run forwards parallel with it to be inserted 
into the rami. These apparently bring about restricted biting 
movements which sometimes occur when the entire jaw 
apparatus is not fully exserted, the chief biting movements, 
with the jaws in a state of complete protrusion being pro- 
bably due to the contractions of the striated muscles inserted 
into the bridles. 
A number of muscular fibres arising from the wall of the 
head in the posterior part of its extent about the bases of the 
anterior limbs pass through the coelomic space that underlies 
the brain, partly in relation to its dorsal, partly to its ventral, 
surface, and are inserted into the rami. In addition, retractor 
and protractor strands pass to the apparatus from the walls 
of the head. 
The digestive system is much the same in essentials in 
Histriobdella and in Stratriodrilus. The whole system, 
apart from the jaws and certain glands to be referred to pre- 
sently, consists of a simple tube (figs. 1 to 6) running straight 
through from mouth to anus. The first part, the oesophagus, 
situated in the head, is an extremely narrow tube. The 
second part, stomach, running through the anterior trunk 
region, is much wider. Then follows a very narrow anterior 
part of the intestine running through the reproductive seg- 
