PHYLUM NEMATHELMINTHES 179 



Class Acanthocephala (a kan th6 sef 'a la, thorn head) includes a 

 group, known as "spmy-headed worms," which is absolutely para- 

 sitic in its habits. The adults are from a few millimeters to fifty milli- 

 meters in length and have an elongated, flattened body when found in 

 the intestine of a vertebrate but become distended to a cylindrical 

 shape when removed to some solution outside the body. The protru- 

 sible proioscis is a peculiar and characteristic structure located at the 

 anterior end of the body. It bears numerous recurved hooks or 

 spines, and in many species it is capable of receding into a proboscis 

 receptacle or sheath. There is no digestive tract in this parasite, 

 and its food is absorbed through the surface of the body even 

 though it is covered with a cuticle. A single ganglionic mass con- 

 stitutes the central nervous system. 



The male reproductive organs are the testes and a set of cement 

 glands joining the cirrus which is held in the copulatory bursa at 

 the posterior portion of the body. The bursa is capable of eversion. 

 The sexes are separate, but the female has no permanent gonads. 

 Egg masses develop early and completely rupture to produce a con- 

 siderable number of embryos in the body cavity. Finally the em- 

 bryos are discharged by way of the uterus through the genital pore 

 which is located posteriorly and is the only external aperture of 

 the body. Not only man, but mice, rats, pigs, fish, turtles, and in 

 fact all classes of vertebrates serve as hosts for these animals. 



ASCARIS, A REPRESENTATIVE ROUNDWORM 



Habitat and Behavior 



The animal which is frequently studied as a representative of this 

 phylum is Ascaris lumbricoides which frequents the digestive tract 

 of men and hogs. It is entirely dependent on its host for furnish- 

 ing suitable food and environment. The only time this organism is 

 at the mercy of the elements of nature is during the egg stage when 

 it may remain potent for months or even years if it falls in an 

 environment unsuitable for development. 



External Anatomy 



This is one of the largest nematodes, females commonly reaching 

 a length of from eight to fourteen inches and males averaging six 



