Thread-worms. 65 



to the stomacl , which is a small suctorial muscular 

 cavity, communicating by a straight intestine with 

 an outlet which is not terminal. Beside the common 

 round A scar is lumbricoides, the human digestive canal 

 is the occasional dwelling-place of two other worms, 

 one of which, Oxyuris vermicularis, is a small thread- 

 like worm (fig. 38), the other Trichocephalus dispar^ 

 much more common, has a very slender neck and a 

 thicker body. A species closely allied to the last 

 named is the Trichina spiralis, a minute worm found 

 in the flesh of pigs, calves, &c., which when introduced 

 into the human body, often multiplies rapidly in the 

 voluntary muscles of the system, causing dangerous 

 and even fatal symptoms. 



These worms are as prolific as their fellow parasites, 

 and the early stages of many live for a time in water, 

 from whence they enter into the bodies of their hosts, 

 and in those whose life-history we know, the free and 

 parasitic conditions appear very dissimilar. It has been 

 supposed and with reason that most of the free Ne- 

 matelmians found in stagnant pools are early stages 

 of parasitic species. 



Gordiacca. The horsehair-like thread - worm 

 which is found in rainwater pools is an example of 

 a second order 01" round worms. This remarkable 

 animal begins life as a little larva living in mud or in 

 water pools ; it is armed with boring spines, whereby 

 it pierces into the body of a beetle or other aquatic 

 or terrestrial insect ; here it becomes encysted, and, 

 having grown in this condition to a considerable length, 

 often ten times as long as its host, it becomes free and 

 aquatic and produces its eggs. So rapidly do some 

 F 



