(375). The male apparatus occupies the centre of the body. The testes (Jc), in which the spermatic fluid is secreted, consist of convoluted vessels of small calibre, arranged in close circular folds, and so inextricably involved, that it is difficult to get a clear idea of their arrangement; but towards the middle of the median line they become more parallel, and terminate in two larger trunks (i) (one of which has been removed in the figure), which are enclosed and hidden in the seminal vessels. These great canals, which run side by side in a longitudinal direction, become gradually much attenuated (I), and terminate in the root or capsule of the penis (m.) The external male organ (n) is placed a little anterior to the orifice which leads to the female parts: it is a short spiral filament, distinctly traversed by a canal, and perforate at the extremity, so as indubitably to perform the office of an instrument of intromission.

Anatomy f Distoma. a, anterior sucker and oral orifice; b, alimentary canal.

Fig. 71. Anatomy f Distoma. a, anterior sucker and oral orifice; b, alimentary canal; c, nervous system; d, external opening of female generative apparatus; e, uterine receptacle; f, accessory appendage to ditto; g, oviduct; h, ovary; i, common canal, receiving k, convolutions of testis; I, vas deferens; m, capsule of the penis; n, intromittent organ.

(376). Among the most interesting discoveries of modern times is the establishment of the long-suspected fact that the Trematode Entozoa undergo certain metamorphoses during their development, and those of a most extraordinary and unheard-of character, exhibiting remarkable examples of the phenomenon of alternate generation. It is to the Danish naturalist, Steenstrup *, that science is indebted for the following account of his researches.

(377). Although the best-known species of the numerous family of the Trematoda is the fluke, or liver-worm, of which the anatomical details are given above, similar forms are met with in almost all animals of the four higher classes; and among the lower, theMollusca are equally infested by them.

(378). It might almost be said that in these classes every species is infested by its own fluke; in various animals, moreover, several different species of these parasites have been found, which inhabit either all the organs of the body indiscriminately, or are exclusively confined to one (liver, kidney, bladder), or to a definite part of an organ. Several of these Trematoda, as will be evident hereafter, when young, are not connected with any viscus, but enjoy the power of free locomotion in water, externally to the animal which, in their future state as Entozoa, they infest. In their free condition they are provided with a locomotive apparatus, usually a tail of moderate length, by the waving movement of which the creature propels itself through the water, like a tadpole, to which, in its external form, it is not dissimilar, though almost of microscopic dimensions. In this larval state the Trematode worms have long been known to naturalists under the generic name of Ger-earia; but although it was well established that this form was not a permanent one, it was not until the researches of Mtzsch, Siebold, and Steenstrup revealed the true nature of the changes through which they pass, that we arrived at any satisfactory knowledge of their remarkable history.

* Ueber den Generationswechsel in den Niederen Thierklassen, translated by the late Mr. Henfrey, in the publications of the Kay Society, 1842.

(379). A Cercaria, supposed by Steenstrup to be the Cercaria ecli'i-nata of Siebold (fig. 72, l), is found by thousands in the water wherein specimens of the large freshwater snails, Planorbis cornea and Limnceus stagnalis, have been kept. The body of this species of Cercaria is usually of a more or less elongated-oval form, which, however, it is constantly changing, assuming, during its movements, every outline, from the circular figure which it has in the fully contracted state, to the linear form that it presents when its body is fully extended; it is furnished, moreover, with a triangular head, at the apex of which is situated the oral orifice, surrounded with an apparatus of spinous teeth; and a ventral sucker is visible, situated upon the inferior surface of its body; while internally traces of viscera are discernible (as represented in the figure), the nature of which is not clearly made out.

(380). The swimming movement of these Cercariae is very characteristic: in performing it, the animal curves its body together into a ball, by which the head is brought near to the caudal extremity, and at the same time the elongated tail strikes out right and left into various sigmoid flexures. In this way they may be seen swarming about the water-snails in great numbers. After swimming about the snails for some time, they affix themselves, by means of their suckers, to the slimy integument of those animals, and all their movements upon it are readily perceived with a good glass. On examining, with a sufficient magnifying power, a portion of the skin of the snail with several of the Cercariae adhering to it, it will be seen that all the efforts of these creatures are directed to the inserting of themselves deeper into the mucous integument, and to the getting rid of the tail, which is no longer of any use to them as an organ of locomotion; in this, after violent efforts, the Cercaria at length succeeds, and the now tailless animal assumes so completely the appearance of a Distoma or fluke, that it could not fail of being recognized as belonging to that genus, in case it were met with in this condition in the viscera of other animals.

However, it undergoes a further remarkable transformation before it becomes a true Entozoon in the common acceptation of the word.

1. Cercaria echinata? of Siebold. 2. Distoma pupa, or Cercaria.