NEMATODA. 685 



is hatched, and becomes sexual (Oxyuris, Trichocephahis 1 ) ; (9) the larva 

 lives in Insecta, the sexual animal in earth or water (Mermis] ; (10) the 

 larva lives encysted in one animal and is transferred with it to a second 

 animal (Ollnlanus, from the Mouse to the Cat 2 ; Cucullanus elegans, with 

 Cyclops to the Perch ; Spiroptera obtusa, with the Meal-worm to the Mouse ; 

 Filaria rhytipleurites, 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 con- 

 taining embryoes are coughed up, the embryo acquires the power of move- 

 ment 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,\h.z 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. cristatus). 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 cavi- 

 ties 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 46 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 vermicular is, the common round Worm, the 

 embryoes of which are set free in the stomach, pass through their stages of 



1 The Oxyuris vermicularis lays ova containing at the time of their escape ready formed 

 embryoes. Hence the great ease with which infection not only spreads, but is maintained in the case 

 of this parasite. The embryoes of Ascaris megalocephala are developed more quickly in damp air 

 than in water. The embryoes are set free from their shells if the latter are strewn on moist earth, and 

 they continue to live if supplied with slices of pear, &c. The natural mode of development probably 

 follows a similar course. See Hallez, C. R. 101, 1885. Von Linstow believes \haijulus guttulatus 

 is the intermediate host of A. lumbricoides ; Z. A. ix. 1886. 



2 The embryo of Ollulanus, a viviparous genus, may wander from the intestines of the Cat to its 

 lungs, sometimes with results fatal to the host. After encystation it undergoes fatty degeneration. 

 This is an instance of a parasite going astray. See Stirling, Q. J. M. xvii. 1877. 



It is probable that the life-histories of a very large number of parasitic Nematoda fall under this 

 heading (10) : among others, perhaps those of Dracunculus medinensis and Filaria Bancrofti s. 

 F. sanguinis hominis. The larva of the former inhabits a Cyclops ; of the latter a certain sp. (?) 

 of Mosquito; but the mode in which both are introduced into the human subject is still un- 

 known. 



