(a) The Holostornum alatum. This worm has been found in the small intestine. It is provided with two small suckers; one situated at its anterior extremity, and at the same time serving as the mouth; the other, on the abdominal surface, enabling the worm to fix itself. The body is divided into two parts, of which the anterior is marked either by a constriction, or by a considerable membranous enlargement, and performs the functions of a sucker; whilst the posterior is the thicker, and is cylindrical in shape.

(b) The Spiroptera sanguinolenta, found in tubercles in the stomach and glands of the oesophagus. No ill effects are produced; when in the stomach there is said to be great voracity of appetite.

(c) The Ascaris marginata.

(d) The Trichocephalus depressiusculus.

(e) The Dochmius trigonocephalus.

(f) Taeiae, or tape-worms, of which there are three or four varieties in dogs.

The common "round worm" is cylindrical in form, from four to eight inches long, pinkish in colour, and tapering towards both ends. They sometimes crawl from the bowel into the stomach, and are vomited either singly, or several coiled up into a ball; or they pass downwards and are discharged. The "maw worm" is from half an inch to an inch long, the tail pointed, and the head obtuse and puckered where the mouth is situated. They exist in immense numbers in the rectum, and set up great local irritation.

Worms of the class Cestoidea, to which the teniae belong, have a soft flattened body of considerable length. The head, which is the smallest part of the body, is situated in front, and is furnished with four suckers and a double circlet of hooks, which fix the parasite to the intestinal mucous membrane of its host. The body is composed of numerous segments, or joints, which are continuous with the head and with each other in a backward direction; each joint is an independent creature, the last joints being provided with male and female reproductive organs, and containing fecundated ova.* The most posterior and larger joints, when sexually mature separate one by one, or several of them together, and are discharged into the outer world, along with the faeces, whilst at the same time new joints are being as constantly produced from behind the head. Sometimes the segmental envelope of a free joint is disintegrated whilst still within the bowel of its host, and then the liberated ova are seen as a whitish sandy powder. Expulsion, however, is the rule, and is a necessary step in the full development and propagation of the worm. The expelled segments may be observed to move by the alternate contraction of their longitudinal and transverse fibres, ova being discharged along the trail.

* Dujardin calculates that the Tenia serrata of the dog contains twenty millions of eggs.

The ova are so constructed as to resist influences apparently the most destructive, and hence they retain their vitality in the outer world for an indefinite period. The majority, however, perish in various ways; whilst the minority, after many ups and downs, blown about by the winds and carried hither and thither by streams, gain admission into the body of some animal either on the grass, leaves, etc., which it eats, or in the water which it drinks. By some such passive means they get into the stomach of man, dogs, pigs, sheep, deer, oxen, etc., where they enter upon a higher stage of development, provided their new abode be adapted to that end; for the ova perish in an unsuitable host.

In a suitable body, the tendency of the eggs is to assume the bladder, hydatid, or cystic form, which is really the undeveloped condition of the mature taenia. Each ripe egg encloses an embryo, consisting of a vesicle armed with six small hooks. In the stomach of the proper host, the capsule of the ova, or the egg-shell, breaks, or is digested away, and the contained embryo is set free. By means of its hooklets it bores its way, and thrusts its vesicular body through membranes, vessels, and solid structures; or, reaching a bloodvessel, is passively carried with the blood to different parts of the body. If it reach an unsuitable region it dies and degenerates into a granular or atheromatous mass. According to the species of worm, the liver in man and cattle, the brain in sheep, and the muscles in the pig, are the proper habitats where alone the embryo survives and becomes a cystic worm. In the substance of a congenial resting-place, the embryo becomes enclosed in a cyst, and the head of the future taenia, furnished with four suckers and a double circlet of hooks, becomes gradually developed from the interior of the embryonic vesicle.

There are three forms of cystic worms, each of which is non-sexual, and consists essentially of a taenia head united by a neck to a vesicular body of variable size and shape according to the species; they are the Gysticercus, the Ccenurus, and the Echinococcus. They reside in the bodies of animals which serve as the prey or food of some other animal infested with the mature taenia. Thus, the Cysticercus pisiformis of the hare and rabbit becomes the Taenia serrata in the dog; the Cysticercus tenui-collis found in the pleura, peritoneum, mesentery, etc., of sheep and oxen, becomes a taenia exactly like the T. serrata when the dog eats these parts of the animal's body; the Cysticercus cettulosae of the pig undergoes a similar development; the Coenurus cerebralis of the sheep passes through the same change; and the Echinococcus of man, or domestic animals, which contains innumerable taenia heads, is bred in the dog into a small taenia with only three segments. These worms, in the bladder state, remain quiescent; they cannot escape from their habitat by any effort of their own; and many of them die either of themselves or with their host, without reaching the mature condition. Before they can be developed into the corresponding mature parasite, they must be devoured along with the flesh of their host by some suitable animal; thus the Cysticercus pisiformis is developed into a corresponding taenia in the dog, but not in the cat. In the stomach of a proper animal, the cyst or bladder of the worm is digested away, leaving the head and neck untouched. The head previously inverted within the cyst is now everted through the neck - turned inside out, in fact. The hooklets and suckers on the head fix themselves firmly into the mucous membrane of the intestine; the body, marked transversely into segments, begins to grow backwards from the head; new joints continue to be produced; the mature sexual ones are detached and escape, and thus the wonderful round of development goes on.