212 LOWER INVERTEBRATES. 



readily becomes an epidemic ; the suffering hens should be isolated from the rest and 

 kept carefully cleaned ; it is said the parasites may be dislodged by brushing out the 

 trachea with a feather. 



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

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

 bricoides, belong to the earliest and most familiarly known of parasites. The pin-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, which pass out with the natural evacuations. 

 The A. lumbHcoides 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 Meemithid^ and Goediidje were long placed near to, but apart from, tlie 

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

 certain peculiarities of their anatomy and development. Both Mermis and Gordius 

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

 body cavities of various insects ; but the}- become mature only in the free state, 3Ier- 

 mis living in the earth, Gordius in the water. The habits of Mermis have given rise 

 to the belief in Europe in a rain of worms ; foi- often in summer after a warm rain at 

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

 larvae 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 under discussion, for M. Villot, 

 who has published a long memoir on the subject, 

 does not agree with earlier observers. " The eggs 

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

 „ ,„„ _ .. truded in a delicate cord resemblino- a thread of 



Fig. 19D. — Oordius. _ = 



sewing cotton ; the eggs are very 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 completed. ... In 

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

 appearance from the parent. The newly developed Gordius is about 

 5^^ 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 complex apparatus, which the animal is capable of pro- 

 truding and withdrawing." Meissner observed the larvre of another 

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

 tains that there always follow several successive migrations before the fig. 200. — L-irva 



•' _ _ => _ _ of Gordtus. 



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



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



that a horse hair has fallen in and been changed to a worm. We know better! 



