180 COMPARATIVE ANATOMY CHAP. 



in the veins into the musculature, penetrate the muscle fibres, and there surround 

 themselves with a cyst or capsule which afterwards becomes calcareous (A). 

 Encysted muscle Trichincc cause trichinosis. Men become infected by eating 

 trichinous pork which is uncooked or not sufiiciently cooked. Pigs are much 

 exposed to infection on account of their omnivorous habits. The principal carriers 

 of Trichina, however, are rats which happen to eat the dead bodies of infected 

 animals, and so secure the continued existence of the parasites. 



Strongylid<x : Dochmius (Anchylostoma) duodenalis, with strong oral capsule 

 armed with teeth. Female as long as 2 cm., male half as long. In the human small 

 intestine (Egypt, Brazil, India, the Antilles, Switzerland, Italy, Belgium). Chiefly 

 among labourers in pits, mines, or tunnels. Causes the so - called miner's ansemia. 

 Eustrongylus gigas, female 30-100 cm. long. In the pelvis of the kidneys of the 

 dog and other mammals. Ascaridcc: Ascaris lumbricoides, male up to 25 cm., 

 female to 40 cm. in length. In the human small intestine. Oxyuris vcrmicularis, 

 female up to 1 cm. in length, male half as long. In the human large intestine. 

 Specially common in children. 



The Gordiidcc hold an isolated position among the Nematoda on account of 

 peculiarities of inner organisation which will be spoken of in the anatomical portion. 

 The mouth in adult animals is closed, and the intestine partly degenerated. 

 Gordius aquations, in the adult sexually mature condition free in fresh water. 

 The embryos find their way into insect larvae, where they become encysted. If these 

 larvae are devoured by preying insects which live in water, the embryos develop in 

 the body cavities of these new hosts, escaping into the water as sexual maturity 

 approaches. 30-90 cm. long, 1 mm. thick. 



Order 2. Acanthocephala (Fig. 172, p. 258). 



Mouth and intestine wanting. At the anterior end a protrusible proboscis 

 armed with hooks. Entirely parasitic. Echinorhynchus : E. gigas, in the small 

 intestine of the pig. Larvse in cockchafer grubs. 



CLASS III. Annulata (Segmented Worms). 



Body elongated, cylindrical, or more or less flattened dorso-ventrally. Skin soft, 

 or with more or less hard and rough chitinoid cuticle. Marked metamerism or 

 segmentation of the inner organs ; generally also outwardly visible. The body cavity, 

 with the exception of Hirudinca and Myzostomidoe, well developed. The blood- 

 vascular system generally well developed, seldom quite degenerated. The intestinal 

 canal runs mostly in a direct line from the mouth to the terminal anus. The 

 nervous system consists of a brain, an cesophageal ring, and a usually distinctly 

 segmented chain of ventral ganglia. The excretory system (wanting in Myzostom/t) 

 consists of segmentally arranged paired nephridia. Nephridia often perform the 

 function of conducting the sexual products to the exterior. 



Order 1. Hirudinea = Discophora (Leeches). 



Body externally ringed. A definite number of consecutive rings correspond 

 with an internal segment. Round the mouth an oral sucker, at the hinder end 

 of the body under the anus a ventral sucker. Skin soft. Setse wanting. Intes- 

 tine mostly with paired lateral diver ticula. Body cavity degenerated and com- 

 municating with the well-developed blood-vascular system. Numerous pairs of 

 nephridia segmentally arranged (looped canals), which are not used for conducting 

 the sexual products to the exterior. Hermaphrodite : testes in several pairs, seg- 



