NEMATODA 215 



animals are, with a few exceptions, unisexual, the females being larger 

 than the males, with a genital pore near the middle or towards the for- 

 ward end of the body. The genital organs are simply formed tubular 

 structures. In the male the testis is usually a single tube which 

 communicates with the rectum. Anal papillae are often present; long 

 spicules also usually project from the anus of the male by means of 

 which it attaches itself to the female; in the Strongillidae and other 

 families an expansion of the hinder end called the bursa also serves the 

 same purpose. The spermatozoa are short and tailless. In the female 

 two genital tubes are present which meet near the external opening. The 

 distal ends of these are ovaries and produce the eggs while the proximal 

 ends act as uteri. The eggs of the various species have a characteristic 

 appearance by means of which the animals may be identified (Fig. 345). 

 Many nematodes are viviparous, the 

 young animals developing in the uterus. 

 Habits and Distribution. Nematodes 

 are mostly active animals which move by 

 a peculiar whipping motion of the body. 

 With the exception of certain minute 



forms which lead a free life in fresh . Jiff- 345 Eggs of various nema- 



todes (from Ward). A, Ancylos- 



and salt water and in decaying organic * oma duodenaie ; B, Necator amer- 



icanus ; C, Ascaris lumbricoides ; 



matter, they are internal parasites of PI l Asoari s canis ; E> T . ri churis 

 J trichiura ; F, Oxyuris vermicularis. 



animals and plants, being among the 



commonest parasites of man and the domestic animals. Like most para- 

 sites, many pass through a metamorphosis in their youth and may live 

 in two different hosts. 



History. Some of the commonest nematodes which 'are the cause of 

 disease in man and his domestic animals have been known for a very long 

 time and were much studied by the early zoologists. In 1808 Rudolphi 

 created the orders Nematoda and Acanihocephala, and in 1851 Vogt 

 formed a class of these two orders and the Gordiacea, which he called the 

 Nematelmia. The most active investigators of nematodes at the present 

 time in this country are the United States Department of Agriculture and 

 the Marine Hospital Service. 



The class contains about 15 families and several thousand species. 



Key to the families of Nematoda here described: 



<&! Mostly non-parasitic nematodes. 



&! Marine and free-swimming nematodes 1. ENOPLIDAE 



6 2 In fresh water or in the ground ; a few parasitic in animals and plants. 



2. ANGUILLULIDAE 

 a z Parasitic nematodes. 



\ Parasitic in invertebrates ; mouth with 6 papillae 3. MERMITIDAE 



6 2 Parasitic in vertebrates. 



