160 JSfemathelminthes. 



There is usually no pronounced metamorphosis, although the young 

 one sometimes differs from the adult. Asexual reproduction does 

 not occur. Most Nematodes are parasitic; some, mostly small 

 forms, are, however, free-living, in fresh-water, damp earth, or in 

 the sea, some in decayed substances or living plants. Many of the 

 parasitic forms live in different hosts, at different periods, or are free 

 for one period, parasitic for another. The habits of these animals 

 are of peculiar interest. 



. 1. The Oommon Bound-worm (Ascaris), often of considerable size, 

 anteriorly three prominent well-developed lips, forming a conical projection 

 marked ofE from the rest of the body. The human Ascaris, A. lumbricoides is 

 frequent in the small intestine, especially numerous in children, and then not 

 without danger: also in Pigs. It probably feeds upon intestinal mucus, not 

 upon blood. According to recent observations, infection is simply caused by the 

 ova, which pass out from the host with the excreta, chancing to enter the mouth. 

 On reaching the stomach the shell is dissolved by the gastric juice,* so that the 

 embryos are set free. The female may attain a length of 40 c/m., the male of 

 25 c/m., both are usually about half as long. A. megalocephala of the Horse is 

 somewhat larger than A. lumbricoides ; A. mystax of the Cat and Dog is consider- 

 ably smaller, the female 12 c/m., the male 6 c/m., and is easily recognised by the 

 wing- or ridge-like fold of skin on either side of the anterior end. 



2. The M a w - w o r m ( Oxyuris vermicularis) . With three rudimentary lips ; 

 at the anterior end, dorsal, and venti-al longitudinal folds of skin. Female with 

 thin, pointed, elongate, spike-like hind-end, 1 c/m. long ; male without this " tail," 

 smaller and less common than the female. Common in the large intestine of man 

 (especially childi-en), feeding on its contents, often present in very large numbers, 

 and then causing serious suffering. Infection probably takes place in the same 

 way as with Ascaris. A much larger species (0. curvula) in the csecum of the 

 horse. 



3. The Strongylidse (genera Strongylus, Eustrongylus, Bochmius), 

 are especially characterised by the presence of a cup-shaped bursa, surrounding 

 the cloacal aperture in the male, which serves as an organ of adhesion during 

 copulation, and is supported by radial rib-like thickenings (Fig. 123 A). Spicula 

 are also present as usual. Most of the StrongyHdse are blood-suckers; the 

 mouth is large, and furnished with chitinous teeth or spines. 



(a) Eustrongylus gigas, the female may be 1 m. long (12 m/m. thick), the 

 male "3 m. In the pelvis of the kidney {i.e., the anterior widened part of the 

 irreter) in the Dog, Otter, Seal, etc., very rare in Man. Life history unknown. 



(6) Dochmius duodenalis (Fig. 123), the female may be 2 c/m. long, the male 

 1 c/m. Mouth with strong hooked teeth ; a very dangerous blood-sucking parasite, 

 living in the small intestine of Man. In the tropics and in warm climates (Brazil, 

 Egypt, Italy) ; also farther north, e.g., in some mining districts of Germany 

 (" Egyptian Chlorosis "). The ova leave the parent and its host, to undergo then- 

 development in damp earth or quagmires, where the larvae live for some time. 

 Then they encyst (the capsule has the elongate form of the animal, and is pro- 

 bably a loosened cuticle), and in this state are probably introduced with drinking- 

 water, or in some such way. 



(c) Strongylus armatus, the Palisade Worm (the latter name comes from 

 a row of chitinous spicules along the edge of the mouth), female to 5 c/m., male 

 2 — 3 c/m. Very frequent in the large intestine (especially in the csecum) of the 



* The egg-shell, just as in most other intestinal parasites, cannot be dissolved in 

 the intestine, the ova must pass through the stomach for this to be effected. 



