TEEMATODA. 143 



vessels of small calibre, arranged in close circular folds, and so inex- 

 tricably involved, that it is difficult to get a clear idea of their arrange- 

 ment ; 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. 



(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 follow- 

 ing 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 con- 

 nected 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- 

 caria ; 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. 



