Fig. 42.—Liver fluke of twice the natural size; a, mouth; b, penis; c, digestive tube; d, abdominal sucker.
We may here cite as an example of this class of parasites the Distomum hepaticum, or liver fluke; this species is the most interesting to us of all the genus; it attains the size of a moderate leech, and habitually resides in the liver of the sheep. In order to discover it, we have only to examine a fresh liver. They are usually found in the biliary canals, where they move about like planariæ. It is always of a deep colour, and is doubtless introduced in the state of cercaria, when the animal is drinking. M. Willemoes-Suhm supposes that the Distomum hepaticum has for a vehicle a small snail, the Limax agrestis, which the sheep swallows with the grass on which it
feeds. Its principal abode is in the ruminants and only casually in man. It is said to be unknown in Iceland. The Distomum lanceolatum has also been found in man.
Dr. Bilharz, the pupil of Siebold, discovered in the year 1851, on man, a parasite in every respect remarkable. It belongs to the family of the Distomidæ, and on account of its peculiarities, it has been made into a genus under the name of Bilharzia. It is found in Egypt, and lives in the vena portæ and in all its ramifications in man. According to Bilharz, this distomian is diœcious, the male being of considerable size, the female slender and delicate, which fact does not agree with the usual characteristics of diœcious animals. At least half of the Fellahs and Copts suffer from these parasites; these worms, at the period when they lay their eggs, proceed from the vena cava to the veins of the pelvis, and after having produced very grave consequences, they are at last evacuated with the urine.
Another distome was also found by Bilharz in the intestines of a young Egyptian boy.
The largest known distome inhabits the liver of the Balenoptera rostrata, the little whale of thirty feet in length, which is regularly met with on the coast of Norway. The intestines of the ordinary seal often contain a very curious distome, which was first observed by Rudolphie, the D. acanthoides. The seal is also infested by the Distomum cornus, which some have incorrectly preferred to place in the genus Amphistoma.
Besides the distomes which inhabit the liver, there are found but few in the mammalia, except in the Cheiroptera: these insectivorous animals have their
intestines literally full of these parasites. We have noticed the species which regularly frequent our bats, and it only remains to discover the insects by means of which they are introduced; for it is probable that these insects are infested by cercariæ during the time that they inhabit the water. Larvæ and their parasites ought to be carefully studied in the localities where bats abound.
There are few birds, especially among the grallæ and the palmipedes, which do not enclose in their intestines a certain number of distomes. The same may almost be said of reptiles and batrachians, but it is especially in fishes that their number is greatly increased. We may say that there is no fish which does not nourish some of these trematodes. Among a portion of these, the cycle of evolution and transmigration is perfectly known; we may instance the Distomum nodulosum. This worm inhabits the intestines of the perch.
The scolex, as well as the cercaria, has its particular characters, and we have long since found the latter in a fresh-water mollusc, the Paludina impura. The cercaria is easily recognized by the presence of two particular folds at the base of the buccal bulb, and by the transparency and the form of the extremity of the urinary apparatus. In the adult distome, this same part of the urinary apparatus encloses large vesicles with very distinct partitions.