Mons. Willemoes-Suhm mentions a distome which at the time of its cercarian evolution lives freely in the water, and attaches itself by its sucker to the larvæ of worms or copepod crustaceans, and then lodges in their dejecta without encysting itself. This is the _Distomum ocreatum_ of the herring, according to Professor Moebius. Mons. Ulialnin found in the bay of Naples another free distome, which is also attached by its ventral sucker to certain copepods, and which becomes the _Distomum ventricosum_ inhabiting many kinds of fish. Any one who wishes to make observations on distomes in the state of 198 cercariæ has only to examine some fresh-water molluscs, either the Limneæ or Planorbes found in ponds; as he tears the animal to pieces on the stage of a simple microscope, he will not fail to perceive a multitude of struggling and wriggling tadpoles. Their tails twist with each other, furl up, extend, and describe arcs of circles, as if we had a nest of serpents under our eyes. Each species of distome has it own cercariæ, which are scattered among as many different inferior animals. Birds and fishes become infested by them in consequence of eating these animals. [Illustration: 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 199 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 dioecious, the male being of considerable size, the female slender and delicate, which fact does not agree with the usual characteristics of dioecious 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.