Biology

In their adult stage, the tapeworms inhabit almost exclusively the alimentary canal of vertebrate animals, with but few exceptions the small intestine, and a few species select definite parts of it. A small number of Rhynchobothriidæ of marine fishes live apparently always in the stomach, while in rays and sharks the spiral intestine is their exclusive site. Bothriocephali generally attach themselves with their head on to the appendices of the pylorus of fishes; other species (Hymenolepis diminuta) occasionally fix their head in the ductus choledochus, and this is more frequent still in the tapeworms of the rock badger (Hyrax), which occasionally penetrate entirely into the biliary ducts. Stilesia hepatica, Wolffh., has so far only been found in the bile-ducts of its host (sheep and goat, East Africa).

In the disease of sheep induced by Cestodes, the worms have been observed also in the pancreas. Specimens found in the large intestines were probably being evacuated.

The Cestodes are looked upon as fairly inert creatures, this opinion having been formed by observing their condition in the cold cadavers of warm-blooded animals. Actually, however, they are exceedingly active, and accomplish local movements within the intestine, for they have been found in the ducts communicating with the bowel, or in the stomach, and may even make their way forward into the œsophagus.

They also invade other abdominal organs through abnormal communications, or through any that may be temporarily open between the intestine and such organs; they thus reach the abdominal cavity or the urinary bladder, or they work their way through the peritoneum.

They produce changes in the intestinal mucous membrane at the place of their attachment, the alterations varying in intensity according to the structure of the fixation organs. The mucous membrane is elevated in knob-like areas by the suckers; the epithelial cells become atrophied or may be entirely obliterated. Dipylidium caninum bores into the openings of Lieberkühn’s glands with its rostellum, dilating the lumen to two or three times its normal size, while the suckers remain fixed between the basal parts of the cells. Species with powerful armatures penetrate deeper into the submucosa, and some that are not provided with exceptionally strong armatures, or are even unarmed, may be actually found with the scolex embedded in the muscles of the intestinal walls or even protruding beyond (Tænia tetragona, Mol., in fowls, etc.). Other species, again, even cause perforation of the walls of the intestine of their hosts.

It is generally assumed that tapeworms, which almost without exception live in the gut of vertebrates, get their nutriment from the gut contents, which apparently they absorb through the whole body surface (cuticular trophopores). In favour of this view is the existence of fat drops in the proglottids, the identity in colour in certain forms between that of the fresh worm and the gut contents and the passage of certain substances derived from medicines (iron and mercury preparation) into the worms in the gut, etc. Whether the suckers are concerned in the absorption of nutriment and to what extent is still questionable.

The length of life of the adult tapeworm certainly varies; as a rule it appears to last only about a year; in other cases (Ligula) it averages only a few days, but we are likewise aware that certain species of Cestodes of man attain an age of several or many years (thirty-five). The natural death of Cestodes often appears to be brought about by alterations in the scolex, such as loss of the hooks, atrophy of the suckers and rostellum, finally the dropping off of the scolex; it is unknown whether a chain of segments deprived of its scolex then perishes or whether it first attains maturity. It has already been mentioned that in a few species the foremost proglottids are transformed into organs of fixation on the normal loss of the scolex.

Abnormalities and malformations are encountered relatively frequently in the Cestodes—such as abnormally short or long segments; the so-called triangular tapeworms, which—if belonging to the Tæniidæ—always possess six suckers; often also club-shaped segments occur between normal ones, or there may be a defect in one segment or in the centre of a number following one another (fenestrated segments); bifurcated chains of segments have likewise been observed, as well as incomplete or complete union of the proglottids, abnormal increase of the genital pores, reversion of the genitalia. Besides the above-mentioned increase of the number of suckers on the scolex (in Tæniæ), there may be a decrease in the number; in other cases the crown of hooks may be absent, or abnormally shaped hooks may be formed.

Classification of the Cestoda of Man.

Order. Pseudophyllidea, Carus, 1863.

Scolex without proboscis or rostellum. Head “stalk” absent.

Scolex never with four, generally with two (or one terminal) bothria. Vitellaria numerous. Uterine opening present. Genitalia do not atrophy when uterus is developed. In large majority of proglottids eggs (or, if formed, their contents) are at the same stage of development.

Family. Dibothriocephalidæ, Lühe, 1902.

Syn.: Diphyllobothriidæ, Lühe, 1910.

Genitalia repeated in each proglottid (polyzootic Cestodes). Ventral and dorsal surfaces flat. Cirrus unarmed. Cirrus and vagina if non-marginal open on the same surface as the uterus. Uterus long, convoluted, often forming a “rosette,” never dilates into a uterine cavity. Eggs thick shelled, operculated, constantly being formed in mature proglottids.

Sub-family. Dibothriocephalinæ, Lühe, 1899.

Syn.: Diphyllobothriinæ, Lühe, 1910.

Segmentation distinct. Scolex unarmed, elongated, sharply separated (generally by a neck) from the first proglottis. Cirrus and vagina open ventrally. Genital pores non-alternating. Vas deferens surrounded by a muscular bulb. Receptaculum seminis large, sharply separated from the spermatic duct.

Order. Cyclophyllidea, v. Beneden.

Four suckers always present. Uterine opening absent. Vitellarium single. Genitalia atrophy when uterus is fully developed.

Family. Dipylidiidæ, Lühe, 1910.

Rostellum if present armed. Suckers unarmed. Uterus breaks up into egg capsules. Paruterine organs absent.

Family. Hymenolepididæ, Railliet and Henry, 1909.

Segment always broader than long. Genitalia single. Longitudinal muscles in two layers. Genital pores unilateral. Testes one to four. Uterus persistent, sac-like. Eggs with three shells.

Family. Davaineidæ, Fuhrmann, 1907.

Rostellum cushion-shaped. Armed with numerous (sixty to several thousand) hammer-shaped hooks in two (rarely one) rows.

Sub-family. Davaineinæ, Braun, 1900.

Suckers armed. Uterus breaks up into egg capsules. Paruterine organs absent.

Family. Tæniidæ, Ludwig, 1886.

Suckers unarmed. Uterus with median longitudinal stem and lateral branches. Female genitalia at the hind end of the proglottis. Genital pore irregularly alternating. Testes numerous in front of female genitalia. Ovary with two lobes (wings). Vitellarium behind the ovary. Embryophore radially striated.