PHYLUM NEMA THELMINTHES 



181 



meat whenever found. Even infected meat, however, would not be 

 dangerous if it were thoroughly cooked. An undesirable practice which 

 formerly prevailed on farms in some localities was to leave dead animals 

 exposed where living animals could eat them. It is better to burn such 

 carcasses or to bury them so deep that animals cannot reach them. 



213. Filaria. — The nematodes known as filariae include a number of 

 species which affect both man and domestic animals, particularly in the 

 tropics. The most injurious type is Wuchereria hancrofti (Cobbold), 

 which is a threadworm living in the lymphatic system of man. The 

 larvae of this parasite are carried about in the blood, retreating to the 

 center of the body in the daytime but at night 

 migrating to the peripheral blood vessels in the 

 skin. At times the adults exist in such num- 

 bers as to obstruct the passage of the lymph 

 and cause a swelling of the limbs and other 

 parts of the body, which suggests the name 

 of the resulting disease, elephantiasis. At 

 night, when the larvae are active and are in 

 the peripheral vessels, they are sucked up by 

 mosquitoes. They continue their development 

 for a time in the mosquito's body and when the 

 mosquito bites another person are transmitted 

 to him. Elephantiasis is a very serious disease, 

 particularly in the South Sea Islands. 



214. Hairworms, — A worm which is popu- 

 larly known as the horsehair snake, and which 

 is believed by many ignorant of zoology to be 

 produced in water from the hairs of horses, is by 

 most authors placed in this phylum. The class 

 to which this worm belongs, known as the ^^'Y ^ «?«^^'"^'^ °^ Gordius. 

 Gordiacea, and of which the type genus 



is Gordius, includes several forms, some occurring in fresh water 

 and others being marine. The hfe histories of these is not well 

 known. The egg is laid in the water and from it hatches a larva 

 with a large proboscis and hooks at the anterior end. Using these in 

 boring, a larva has been known to force its way into the body of an 

 aquatic insect. Full-grown larvae have been found in beetles, grass- 

 hoppers, and crickets, and larvae in different stages of development in 

 spiders, earthworms, and snails, but how they got into these hosts does 

 not seem to be known. The fully developed larva escapes from the host 

 into water, where it matures and where it may be found, often coiled 

 about among the leaves of aquatic vegetation (Fig. 88). 



215. Spiny-headed Worms. — Another class of worms which may also 

 be placed in Nemathelminthes is that of the spiny-headed worms, or 



Fig. 88. — Portions of two 

 branches of water milfoil 

 {Myriuphyllum) in which is 



