Introduction to Animal Morphology. 187 



CHAPTER XXVII. 



CLASS 4. — ACANTHOCEPHALA {Rudolpki). 



Cylindrical, dioecious parasites, with a protrusible 

 proboscis, armed with transverse or oblique rows of 

 strong recurved hooks, resembling that of the strongy- 

 loid genus Hystrichis ; at its base is a sheath, within 

 which it may be retracted by four lateral retractor 

 muscles. This organ is attached to, and often im- 

 bedded in, the wall of the intestine of the host. The 

 yellow cuticle is sometimes armed with chitinous 

 hooks, and always traversed by pore-canals, whereby 

 nutrition takes place. Beneath it and the proto- 

 plasmic dermis is a layer of striped, circular, and 

 longitudinal muscular fibres. Three laminae of muscle 

 lie in the wall of the proboscis sheath. There is no 

 mouth nor intestine. A nerve ganglion lies at the base 

 of the proboscis, to which it gives two branches. 

 Two or more lateral nerves pass backward. In the 

 muscular layer are many branching* canals, with two 

 chief lateral stems, extending forwards. These have 

 no special walls, so the granule-holding, often red, 

 fluid within them is moved by the contraction of the 

 body and the motions of the proboscis. Possibly this 

 may be a nutritive apparatus like the gastro-vascular 

 canal of Ccelenterata. The lateral stems end in a 

 circular vessel in the neck, from which, branches pass 

 to the proboscis. With the stems of these probosci- 

 dean vessels canals communicate from two band-like 



* Not always much branched. In E. porrigens the branches appear 

 in some young specimens to be few, straight. 



