212 LOWEll INVERTEBRATES. 



readily becomes an epitleiiiic ; tlic .suft'eriiii;' liens should be isolated from the rest and 

 kept carefully cleaned ; it is said the j)arasites may be dislodged by brushing out the 

 trachea with a feather. 



The AsCAEiD^ also include several species, long known as human parasites; two 

 of these, the pin-Avorm, Oxyuris vermicularis, and the large round-worm, Ascaris lum- 

 bricoides, belong to the earliest and most familiarly known of parasites. The j)in-worm 

 is extremely common, and very generally distributed over the world. The eggs are 

 so light that they are easily scattered about, and when swallowed along with some 

 other object, they develop in the intestines to the adult worm. The habits of 

 civilized life diminish the danger from these parasites, especially as modern systems 

 of sewage constantly remove the eggs, wliich pass out with the natural evacuations. 

 The A. lumbricoides grows to a large size, six or seven inches ; is cylindrical, but 

 tapers towards the head and somewhat towards the tail ; the color is reddish brown. 

 The animal is a parasite of the small intestine. The eggs pass out, and in water or 

 moist earth await the completion of embryonic growth, but how the larva reaches the 

 place of its final development has not yet been ascertained. 



The Mermithid-e and Gordiid^ were long placed near to, but ajiart from, the 

 Nematoda proper, but the best opinion now groups them with this class in spite of 

 certain peculiarities of their anatomy and development. Both Mernih and Gordlus 

 are commonly known as hair-worms, and are found while sexually immature in the 

 body cavities of various insects; but they become mature only in the free state, Mer-- 

 mis living in the eai-th, Gordiiis in the water. The habits of Mermis have given rise 

 to the belief in Europe in a rain of worms; for often in summer after a warm rain at. 

 night they swarm to the surface, and appear to have been indeed rained down. The 

 larvaj are parasitic in caterpillars, but exactly how they gain entrance to the body of 



their insect host is not known. The metamorphoses 

 of Gordius are still mider discussion, for M. Villot, , 

 who has published a long memoir on the subject, 

 does not agree with earlier observers. "The eggs; 

 of Gordius variahilis^'' writes Dr. Leidy, " are ex- 

 truded in a delicate cord resembling a thread of 

 sewing cotton; the eggs are vety minute, and as 

 the parent may be a foot long, it is able to produce an enormous number of young, 

 Leidy estimates over six million. The development of the young is readily observed 

 from day to day, and it takes about a month before the process is coin])k'ted. ... In 

 about four weeks the Gordius escapes from the egg, totally different in 

 appearance from the parent. The newly developed Gordius is nl)out 

 jjjy of an inch long. The body is constricted just posterior to the 

 middle, so as to appear divided into two portions. The anterior 

 thicker portion of the body is cylindrical, distinctly annulated, and 

 contains a comi)lex apparatus, which the animal is capable of pro- 

 truding and withdrawing." Meissner observed the larvre of another 

 species p.enetrate the larvae of May flies and caddis flies. Villot main- 

 tains that there always follow several successive migrations before the fio. sno.— Larva 



, . , , ^ of Gordius. 



worm reaches its last host, but his observations are not convincing. 



The adult Gorditts is sometimes found in the water, and country superstition affirms 



that a horse hair has fallen in and been chnnsrod to a worm. W: know better! 



