PARASITIC ROUNDWORMS 543 



recurved hooks which are arranged in rows. One can distinguish 

 both longitudinal and circular rows and as the hooks alternate 

 they form a quincunx pattern. The number, form, and arrange- 

 ment of the hooks are again diagnostic features. Usually the hooks 

 are strongly recurved but they may be almost straight and often 

 the form varies from tip to base of the proboscis. The form of the 

 root is also subject to variation in different species. In a few cases 

 the hooks differ on the dorsal and ventral sides of the proboscis. 



In most species a neck intervenes between the proboscis and the 

 body proper. It is nearly always unarmed and usually short. At 

 times it is externally very sharply marked off from the body or 

 again difficult to distinguish. Internally a cuticular fold or sep- 

 tum divides the hypoderm of the proboscis and neck completely 

 from that layer in the body. The circular insertion of a retractor 

 muscle at this point also separates these regions from each other. 



The body proper forms the major part of the animal. It is 

 usually unarmed but may bear small spines of definite form and 

 arrangement on some portion of the external surface. 



The body wall has on the exterior a thin cuticula which is not 

 conspicuous as in nematodes. The subjacent hypoderm possesses 

 in one group a few very large and prominent nuclei which were 

 seen by early investigators though their true nature was not divined. 

 These nuclei usually show as swellings or prominences on the sur- 

 face. In most Acanthocephala, however, the hypoderm has many 

 small nuclei which cannot be seen on casual observation. Two 

 elongate organs, the lemnisci, are projections of the hypoderm 

 posteriad into the body cavity. They originate at the line be- 

 tween neck and body proper and vary in size and form in different 

 species. Their function is unknown. The body wall contains a 

 system of lacunae which is conspicuous both in living and pre- 

 served specimens as two longitudinal vessels with smaller anasto- 

 moses usually numerous and irregular. 



The proboscis sheath, usually a closed muscular sac, is attached 

 at the base of the proboscis, or rarely inside that organ. The pro- 

 boscis can be inverted into the sheath. The brain lies within the 

 sheath concealed between the retractor muscles. Its precise loca- 

 tion may be determined by the retinacula, a pair of nerve cords 



