Fig. 46.—Sporocyst of Amphistomum sub-clavatum from the Cyclas cornea.
A worm very common in the intestines of the green frog is known by the name of Amphistomum sub-clavatum. Its cercariæ are usually found in an acephalous mollusc, known by the name of Cyclas cornea. That which distinguishes the scolices of this species is the great contractibility of the external membranes of the young individuals; they lengthen, they shorten, they swing to the right and the left, describing a semicircle on the anterior half of the body ([Fig. 46]). We represent side
by side the cercaria of this amphistome, and the adult and sexual amphistome, as it is found in the intestines of the frog.
Constantine Blumberg has recently published an interesting memoir on the structure of the Amphistomum conicum.
A beautiful trematode worm, known by the name of Hemistomum alatum, whose antecedents have not been ascertained, lives usually in the intestines of the fox. It is about four or five millimètres in length (about ·17 in.). Many birds harbour Holostomes which belong to the same group, the first state of which is not yet known. The Holostomum macrocephalum is common in the intestines of rapacious birds; it is from five to seven millimètres in length (about ·23 of an inch).
We close the history of trematode worms by giving the figure of a beautiful one known under the name of Polystomum, which lives in its adult state in the bladders of frogs ([Fig. 48]). Interesting observations have recently been made on the manner in which they are introduced into the bladder.
The worms which naturalists call Cestoïds, or Cestodes (which means, like ribbon or tape), have for their type the tape-worm known by every one. They are very abundant in many animals, are found in almost every class of the animal kingdom, and are almost as common as the distomians, of which we have just spoken. They are introduced into animals which are vegetable-feeders, by means of water and plants, and into carnivorous animals by their prey. The tape-worms of the herbivora lay eggs like the others, but their embryos have, as soon as they are hatched, a ciliary covering which allows them
to live and move about in the water. Those of beasts of prey are entirely different; it is by means of the prey that they enter their hosts. Each carnivore has its own worms, as it has its own prey which introduces them.