The mode of life in the Nematoda is very variable, and no less than thirteen distinct modifications of development are enumerated by von Linstow as follows1: - (I) The embryo developes directly into the sexual animal, and inhabits fresh, salt, or brackish water, earth, plants, putrefying substances, e. g. Enoplus and many other genera; (2) the larva lives in earth, the sexual form in plants, e. g. Tylenchus Tritici in the ear of wheat, T. putrefaciens in the Onion; (3) the larva lives in worms, is set free by their death and decomposition, and becomes sexual in the earth (Rhabditis pcllio); (4) the sexual worms live in the earth; the fertilised female enters different species of Humble Bee (Bombus) and the Wasp, passes into the coelome, produces ova which hatch out, and the offspring bore their way into the intestine, and so escape (Sphaerularia Bombi); (5) the larva lives in earth; the sexual form in a Vertebrate (Dochmius, Strongylus); (6) the worm lives as a hermaphrodite in an animal, its progeny becomes sexual in earth, and the progeny of the sexual animals enter the animal again (Angiostomum, e. g.

A. nigrovenosum, often called Ascaris or Leptodera nigrovenosa, from the lung of the Frog; Rhabdonema strongyloides from the intestine of Man); (7) a bisexual free form gives origin to a bisexual form parasitic in a Snail (Leptodera appendiculatd); (8) the egg is laid and passes into earth, gives origin to an embryo which is transferred within the egg-shell to an animal, and hypo-blast: that its edges fold over, fuse from behind forwards, leaving a pore - the future mouth (Z. W. Z. xxvi. 1876). A similar mode of closure of the blasto- (gastro-) pore has been noted in A. megalocephala and Angiostomum nigrovenosum. Gotte found an epibolic gastrula in the last-named worm; a stomodaeum but no proctodaeum (Entwickelungsgeschichte der Thiere, i. 1882). He also says that a proctodaeum has been observed by Ganin and Natanson in some Nematoda. Orley found both a stomo- and a procto-daeum in Anguillula aceti, s. Tylenchus oxyphila (Monographic der Anguilluliden, Buda-Pesth, 1880). In certain Oxyurids Galeb states that there is a delaminate Gastrula, and that the whole digestive tract is derived from two ectodermic invaginations (A. Z. Expt. vii. 1878, pp. 323, 368). Judging from structural anatomy alone, both oesophagus and rectum would be said to be respectively a stomo- and proctodaeum.

1 Z. W. Z. xlii. 1885, pp. 715, 716. Von Linstow reckons a 14th-mode, that characteristic of Gordius; see p. 687. post. The heads are kept as given by Von Linstow, but instances have been added and a few slight changes introduced into the wording.

Is hatched, and becomes sexual (Oxytiris, Trickocephalus1); (9) the larva lives in Insecta, the sexual animal in earth or water (Mcrmis); (10) the larva lives encysted in one animal and is transferred with it to a second animal (Ollulanus, from the Mouse to the Cat2; Cticullamis elegans, with Cyclops to the Perch; Spiroptera obtusa, with the Meal-worm to the Mouse; Filaria rhytipleiirites, from the Cockroach to the Rat); (n) the sexual Worm lives in the intestines of a Vertebrate, the female produces young which penetrate its walls and encyst themselves in the muscles (Trichina spiralis); (12) the sexual worm lives in the tracheae of Birds, the ova containing embryoes are coughed up, the embryo acquires the power of movement and is swallowed in food within the egg-shell, quits the shell in the oesophagus and stomach, wanders into the bronchia and air-sacs, and thence, when grown larger, into the trachea (Syngamus trachealis, the cause of Gapes); (13) there are two larval forms; the first lives in water, the second in the lung of an Amphibian, whence it wanders into the intestine and becomes sexually mature (Nematoxys longicauda in Triton alpestris, and more rarely in T. cristatns). It must be noted that in (6) and (7), supra, there is an Alternation of Generations.

The parasitic Nematoda most commonly infest the digestive tract, but they occur also in the lungs, kidney, and urinary bladder of Vertebrata, as well as encysted in various parts of the body. Haematozoa, or parasites living in the vascular system, also occur, e.g. Filaria immitis in the right cavities of the heart of the Dog; and Strongylus armatus, the palisade Worm, in an immature state, is a common cause of aneurism, especially abdominal aneurism, in the Horse and Ass. The species ordinarily inhabiting Man are the following: - Ascaris lumbricoides, the male of which is 4-6 inches long, the female 10-14, inhabits properly the upper and middle part of the small intestine, but may enter the stomach and escape through the mouth or perforate the intestine, and even the abdominal walls, where it may give rise to abscesses; Oxyuris vermicularis, the common round Worm, the embryoes of which are set free in the stomach, pass through their stages of development in the small intestine, enter the caecum, their head-quarters, and are found throughout the whole large intestine; Dochmius (Anchylo-stomd) duodenalis, which inhabits the small intestine, and occurs in Europe, Egypt, Comoro Is., Brazil, and Cayenne, and is the cause of a pernicious anaemia; Filaria Bancrofti s.