IN THE ANIMAL KINGDOM. 95 



As the reproduction of the Trematoda will be afterwards 

 noticed more in detail, in illustration of alternation in the 

 protomorphic stage, it will suffice at present to observe that 

 the recent researches of Van Beneden* go to show that 

 these parasites fall into two very natural groups, distin- 

 guished alike by differences in structure, habit, and develop- 

 ment. Some are ectozoic, that is, parasitic on the exterior 

 of other animals ; they live nearly all on the gills of fishes, 

 and attach themselves by one or even many sucking discs 

 situated at the back part of the body. These species are 

 viviparous, and the young, hatched from the large ova, 

 within the body of the parent, have a development as di- 

 rect as in any other animal. The second group, of which 

 the genus Distoma may stand as an example, live in the 

 interior of the body, and attach themselves by a sucker in 

 the fore or middle part of their body. These are all ovi- 

 parous. The eggs are small and very numerous, and a 

 succession of diverse forms is very constantly interposed in 

 the course of their development. The germ which escapes 

 from the ovum, in the form of a ciliated animalcule, under- 

 goes itself no farther development, but matures in its in- 

 terior, and discharges a tubular sac, furnished occasionally 

 with some rudimentary organs. Gemmae are formed in its 

 interior, and these, when set free by its rupture, are either 

 converted themselves into the typical form, or give origin 

 to others which are so. In the genus Distoma this trans- 

 formation is generally effected by a gradual metamorphosis, 

 the form first assumed being that of a Cercaria or micro- 

 scopic tadpole, which, losing its tail, is eventually trans- 

 formed into the perfect Distoma, during an encysting pro- 

 cess which the parasite undergoes, immediately on penetrat- 

 ing into the tissues of living animals. Sexual organs are 



* See Translation of Van der Hoeven's Abstract, in Annals of Nat. Hist., 

 3d Ser., III., p. 344. 



