HELMINTHES. 47 



oblong, suddenly tapering to a point posteriorly ; sides nearly parallel ; head 

 truncate in front ; neck narrowed, eye dots two, on the narrow part, oblong 

 and white, witli a black dot upon the internal margin ; ventral opening less 

 than one third the entire length from the posterior extremity. It inhabits 

 springs in eastern Pennsylvania, and feeds upon animal matter. When cut 

 in two, each part becomes an entire animal. 



III. CA^ITARIA. 



a. Enterodela. 



1. The Acantliocephala^ which are placed with the Parenchymata by 

 Cuvier, are composed of round •worms represented by the genus 

 Echinorhynchus, the chief character of which is a straight, round, retractile 

 trunk, armed with rows of recurved tooth-like hooks, whicli retain it in 

 place when thrust into the intestines. A small pore is sometimes 

 obser^'able at the end of this trunk, but it is probable that nourislnnent is 

 absorbed by its entire surface. A vascular tube runs longitudinally on 

 each side, and is lost towards the posterior extremity. These noxious 

 animals seem not to have been found in man, although numerous species 

 infest vertebrate animals. They sometimes bore through the intestines, and 

 pass into the cavity of the abdomen, and into other j^arts of the body. 

 Echlnorliynchus gigas {pi. 77, Jig. 39) is from three to iifteen inches long, 

 and infests hogs, especially such as are confined to be fattened. 



b. Enterodela cylindrica. 



1. The Ascaridce include various genera of internal parasites, of which 

 the genus Ascaris is among the best known, and contains slender, round 

 worms, tapering towards both ends. The head is provided with three little 

 valves, between whicli the mouth is placed. The intestine is straight, the 

 vent terminal, and the nervous system bilateral. Ascaris lumhricoides 

 {pi. 77, fig. 41) takes its trivial name from its general resemblance to a 

 lumbricus, or earth-worm. It is white, from six to twelve or fifteen inches 

 long:, and the female is laro;er and more abundant than the male. It infests 

 the small intestines of man, and is frequently fatal to children, in which it 

 sometimes penetrates to the stomach, and even to the mouth. As many as 

 five hundred have been ejected by a child in the course of eight days. 

 It is sometimes discharged from abscesses in the abdomen, and it has 

 rarely been passed from the bladder. This species infests the hog and the 

 ox, and an allied one (once consicTered the same) is found in the horse. 



Owen, some years ago, calculated the number of eggs in a female of this 

 species to exceed sixty millions. Gluge and Mandl found eggs of an 

 Ascaris, without the animal, in the lungs of frogs, supposed to be introduced 

 with the air. 



Oxyuris vermicularis, Linn. {pil. 77, fig. 42), is half an inch long, 

 cylindrical, with the posterior portion subulate. It infests the large 

 intestines of children. 



Trichocephalus dispar {fig. 40) is found with the preceding species, and 

 differs in having the anterior extremity attenuated like a thread, and 

 resembling the lash of a whip, of which the posterior end would be the stock. 

 It is one or two inches long, of which the thick part occupies about a third. 



251 



