We find occasionally between the feathers of some birds tubercles of the size of a pea, and when we open them we see in each two similar worms, placed so that the stomach of one is applied to that of the other; this is the monostome of which we have spoken above. These worms are from three to four millimètres in length (about ·13 in.), and are found in the titmouse, the siskin, the sparrow, the canary, and some other birds. [Illustration: Fig. 45.--Cercaria of _Amphistomum sub-clavatum_.] [Illustration: 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 204 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 205 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.