For detailed technical information concerning the morphology and classification of the parasitic nematodes reference should be made to the well illustrated volume by Warrington Yorke, The Nematode Parasites of Vertebrates. Mention can only be made here of a few forms which are associated with disease in man and the lower animals. One of the most interesting of these is the Filaria bancrofti, a delicate thread-like worm which lives in the lymphatic vessels and is reputed to be the cause of a number of different diseases in man which have been grouped together under the term filariasis (q.v.). Some of these diseases are due to the damage done by the worm to the lymphatic wall, others are at tributable to secondary invasion of these damaged lymphatics by streptococcic germs. The young of this parasite appear in large numbers in the peripheral blood stream at night and disappear during the day time. Manson showed that this rhythmic appear ance was associated with the habits of the intermediate host—a night-biting mosquito—culex fatigans, in the thoracic muscles of which the essential larval development takes place. An allied filarial worm Dirofilaria immitis inhabits the right side of the heart of the dog (Plate, fig. 8). This parasite is especially common in China but occurs also in Italy and South America and is spread by a mosquito. In natives of West Africa worms frequently cause conjunctivitis. They belong to the species Loa loa and are trans mitted from man to man by a day-biting insect of the genus Chrysops. In this case the young worms appear periodically in the blood, but during the day only, in strange compliance with the day-biting habits of the insect vector. Closely allied to the filarial worms are the nodule worms of the genus Onchocerca. These worms produce tumours sometimes of the size of a walnut in the tendinous attachments of muscles. Such tumours are common in man in certain parts of tropical Africa, especially in the Congo.
The young of these worms congregate in the deeper layers of the skin whence they are sucked up by midges of the genus Simulium. The hookworms of man, of which there are two species, are relatively short nematodes measuring about half an inch in length.
They inhabit the small intestine, usually the jejunum, and attach themselves by means of teeth or cutting oral plates. The bites of these worms result in a considerable loss of blood and when numerous worms are present anaemia may result. It has been esti mated that 9o% of the inhabitants in many tropical countries are infested by hookworm. Many of these, however, show few symp toms of disease owing to an acquired tolerance. Related to the hookworm is the gape-worm of poultry. Syngamus trachealis lives, as its name indicates, in the wind-pipe. It is a common cause of death in young chickens and pheasants, partly through the obstruc tion of the air passage, partly from the abstraction of blood upon which the worms feed. Adult birds practically never harbour the worms. Turkeys and starlings are frequently infected by this parasite without suffering to any extent. It is suspected that they are the chief reservoirs of the infection.
Many different species of round-worms, somewhat more dis tantly related to the hookworm, live in the gut and lungs of sheep, goats, cattle and horses. Many of these probably do little harm but the results of massive infection with several species show themselves in general loss of condition and of body weight and in increased susceptibility to bacterial disease.
The Platyhelmia are hermaphrodite, are leaf-like or pear shaped, and, in the case of the Cestoda, tape-like in outline.
Turbellaria are distinguished morphologically by the presence of numerous cilia on the surface of the body. They probably pro vided the ancestor of the trematodes, but are otherwise of little interest in relation to animal parasites and disease. A few para sitize molluscs. The Trematoda and Cestoda are covered with a cuticular skin which may or may not be clothed with spines of similar material. The Trematoda are distinguished morphologically from the Cestoda by the presence of a gut which ends blindly. The Cestoda have derived from the Trematoda and may be re garded as a degenerate branch.
Trematoda and Disease.—The trematodes generally inhabit the alimentary canal. A few forms occur in the bile ducts and in some of the birds an occasional species has adapted itself to live in the conjunctival sac and the generative passages. The highly spe cialized group of flukes to which the bilharzial worms belong live in the blood stream. Other small groups inhabit the mouth, lung and air passages. The flukes give rise to composite eggs contain ing within the shell masses of yolk-cells in addition to the ovum.
The vast majority of flukes, including all those of medical im portance, require for their larval development a mollusc as inter mediate host. The egg, when discharged from the definitive host, usually takes several days or weeks in water to hatch. A ciliated larva emerges which proceeds to its further development by at tacking a specific mollusc. Within this mollusc, larval develop ment proceeds. By asexual multiplication large numbers of infec tive larvae, technically called cercariae, are produced. These cercariae are discharged from the mollusc into water whence they reach the definitive host either (a) by direct penetration of the skin, as in the case of Bilharzia, (b) by encystment on plants which are edible, as in the case of the liver fluke of the sheep, or (c) by penetration and subsequent encystment in edible fishes, in crabs, or in mosquito or other insect larvae, or in other molluscs. For a discussion of the intricacies of trematode morphology and classification the reader should refer to the recent monograph by Poche, Das System der Platoden and to the somewhat earlier writ ings of Looss and Odhner. Of greatest economic importance is Fasciola hepatica which infects the bile-ducts of sheep, cattle, horses and rabbits. The presence of these parasites causes marked thickening of the walls of the bile-ducts and subsequent hepatic insufficiency. In tropical countries the lining of the intestinal wall of sheep, cattle, horses and elephants is frequently eroded by vast numbers of clinging amphistome flukes. A third group calling for specific reference here is that of the schistosomes or bilharzia worms (see BILHARZIASIS). Unlike most flukes these are unisexual and live in the portal system. The eggs are laid by the female in its finer ramifications on the walls of the lower bowel and bladder through which they ultimately work their way to be discharged in faeces and urine. Bilharzia, various species of which parasitize man, dog, sheep, cattle, goats and camels, develop like other flukes in molluscan intermediary hosts. The infective young or cercariae invade man and other definitive hosts by piercing the skin. Preventive measures against fluke diseases are of two kinds: (a) the adult flukes in the definitive hosts and the mollusc car riers of the larval stages can be destroyed by appropriate chemi cals. (b) Infection can be avoided by the heat sterilization of food containing encysted forms and by avoidance of exposure of the skin to infected water.