NEMATELMIA ; NEMATOIDEA. 3 1 1 



are distinct traces of a nervous system. The sexes are always 

 on separate individuals. In the PLATYELMIA, on the contrary, 

 the body is almost always flat and broad, covered by a soft skin, 

 often containing calcareous corpuscles, but destitute of any indi- 

 cations of segments. The intestinal canal has only a single 

 orifice, and the nervous system is far less distinct than in the 

 Nematelmia. These animals are hermaphrodites. 



923. Section I. NEMATELMIA. These worms, which are 

 all parasitic in their habits, form three orders. The GORDIACEA, 

 or Hair-worms, have the body so extraordinarily long and thin 

 as to resemble horse-hairs, and their intestine has no posterior 

 orifice. They are parasitic in the bodies of different species of 

 insects, where they live amongst the parenchyma exterior to the 



intestine. When ma- 

 ture, they quit the bo- 

 dies of their victims 

 and proceed to deposit 

 their eggs in long 

 FIG. 634.-GORDIU8 AOTATICUS. a. tii. chains in the water, or 



in moist situations. If 



the weather be dry at the time of their escape, they often be- 

 come dried up until they form slender horny threads, which arc 

 so brittle as to be easily broken ; but even in this state they re- 

 tain the principle of life, and the occurrence of a shower of rain 

 is sufficient to restore them at once to activity. The young are 

 of course hatched from the eggs, in the water or damp earth in 

 which the latter are deposited, but they soon penetrate into the 

 body of some insects, in the interior of which their development 

 takes place. The NEMATOIDEA are distinguished from the pre- 

 ceding by the existence of an anal orifice, and by their shorter 

 and thicker form. This order includes several species which are 

 parasitic in the intestines of Man. Such are the Ascaris lum- 

 bricoides, well known as the Round-worm, which is of the size 

 and form of a small earth-worm, whence its specific name ; and 

 the Oxyuris rcrmicularis, or Thread-worm, which frequently 

 occurs in great numbers, especially in children, to whom it is 

 exceedingly troublesome. Other species live in closed organs of 



