No. 2.] AJVATOA/V OF BDELLODRILUS ILLUMINATUS. 51 1 



head at the beginning of the pharynx, and extends from the 

 walls of the latter on all sides nearly to the external muscle 

 layers of the head, to which it is bound by numerous radiating 

 fibres (Fig. 10), A few dorso-ventral fibres cover the anterior 

 and posterior faces of both plates. The circular fibres taper 

 more or less from a nucleated middle toward both ends. 



The mechanism of the jaws is seen to be powerful and effi- 

 cient. The muscular plates, with their radiating fibres, regu- 

 late the distance between the two jaws, approximating or 

 separating them as the circular or radial fibres contract in turn. 

 The circular muscles seem sufficiently powerful to bring the 

 jaws together with crushing force. The protractor muscles 

 carry the jaws forward (with a rotary or rocking movement on 

 the muscular pads) against an object of attack, the lower jaw 

 acting with its teeth as a hook, while the powerful retractor 

 muscles of the upper jaw bring its toothed blade with a shearing 

 motion between the ventral teeth. Thus is constituted an 

 efficient pruning apparatus, the chief purpose of which is, I 

 believe, the clipping off of branchial filaments of the crayfish 

 host, from which the blood is then drawn. They are probably 

 also used for mowing down the colonial infusorians which cluster 

 along the borders of the branchial chamber, and remains of 

 which are frequently seen with diatoms, etc., mixed with cray- 

 fish blood in the stomachs of worms examined. 



The jaws mark the beginning of the pharyngeal region, 

 which extends to the oesophagus in the anterior part of the 

 first post-cephalic somite (Fig. 9). The region is distinguished 

 by the great development of muscular tissue, and doubtless 

 functions in the capacity of a suction bulb to increase the flow 

 of blood from the wounded tissues of the host. The structure 

 is similar throughout. A delicate cuticle lines its lumen, thin- 

 ning out and disappearing where the transition from pharyngeal 

 to oesophageal regions takes place. The epithelium is charac- 

 teristic. As has been described above, cell boundaries are 

 seldom clearly distinguishable, but the deeply staining proto- 

 plasm, with its contained nuclei, is crowded, with the exception 

 of a delicate continuous layer, from its normal position at the 

 surface, into irregular processes, which fit plug-like between 



