T/ireadzuorms 65 



to the stomach, which is a small suctorial muscular 

 cavity, communicating by a straight intestine with an 

 anus which is not terminal. Beside this common round 

 Ascaris lumbricoides, the human digestive canal is the 

 occasional dwelling-place of two other worms, one of 

 which, Qxyuris vermicularis, is a small thread-like 

 worm (fig. 38), the other Trichocephalus dispar, much 

 more common in the human caecum, has a very slen- 

 der 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 many of the free Ne- 

 rnatelmians found in stagnant pools are early stages 

 of parasitic species. 



Gordiacea. The horsehair-like thread - worm 

 which is found in rainwater pools is an example of 

 a second order of round worms. This remarkable 

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

 water pools ; it is armed with a boring spine, 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 



