vi ACANTHOCEPHALA ANATOMY 175 



outside of the sheath to the body- wall ; these serve to retract the 

 whole sheath and its contents into the body-cavity of the trunk. 

 The structure of the skin is essentially like that of Neinatodes, 

 but the details are much more complicated. The whole body is 

 covered by a thin 

 cuticle secreted by 

 the epidermis, which, 

 as in the other 

 groups, breaks down 

 and forms a syncyt- 

 iuui called the sub- 

 cuticle. The minute 

 fibrils which pene- 

 trate this layer are 

 much more definitely 

 arranged than in 

 Neuiatodes; the 

 largest of them run 



from without in- FIG. 93. A, Five specimens of Echinorhynchus acus Rud. 

 , attached to a piece of intestinal wall, x 4 ; B, the 



Wards, Others run proboscis of one still more highly magnified. 



concentrically round 



the body. Large oval or spherical nuclei are scattered in the 

 sub-cuticle, which is further honeycombed by a number of lacunae 

 or spaces which are described below. 



Within the sub-cuticular layer is found a sheath of circularly- 

 arranged muscle-fibres, and within this again a sheath of longi- 

 tudinal muscles which do not extend into the proboscis ; this 

 inner layer lines the body-cavity, there being no epithelium 

 within it. In their minute structure the muscle-cells resemble 

 those of Nematodes. 



The canals in the sub-cuticle form a very curious system of 

 anastomosing spaces, in which a clear fluid containing fat globules 

 circulates. The extent to which the system is developed varies 

 in different species, but in all there is a pair of longitudinal 

 canals which are situated laterally, and which give off the sub- 

 sidiary channels in their course. The above description applies 

 to the lacunar spaces in the skin of the trunk ; those of the pro- 

 boscis are quite distinct, and there is no communication between 

 the two sets of spaces ; in fact, the sub-cuticle in which the lacunae 

 are formed is not continuous across the line of junction of the 



