PARASITES OF ANIMALS. 169 



young brood attach themselves to some passing insect and pene- 

 trating- to the interior of its body, remain till sufficiently matured 

 to escape as their parents had done before them ; or, if the eggs 

 are first devoured, they are hatched within the insect, and there 

 developed as in the other case. There are then in the history of 

 the Gordius, two clearly marked periods. The immature mostly 

 passed in the body of its host, and the mature reached after its 

 escape. This is not a solitary instance, but will suffice to exhibit 

 the law of development in this and other genera analagous. 



Thus, at least, some of the supposed cases of equivocal repro- 

 duction are found to be under the operation of the same laws of 

 sexual development. 



If we carefully observe the modes of reproduction exhibited by 

 the Aphides (plant lice), v/3 find them truly surprising — offspring 

 appearing which bear no raeemblance to their immediate progeni- 

 tors — and this fact may hold for several successive generations. 

 But at last individuals appear like those with which we commenced 

 our observations ; and the cycle is produced continuously, the 

 various forms succeeding in the same order. This is called 

 Alternate Generation. 



We find it illustrated in various classes of animal parasites. 

 There may be found in company with certain fresh water molluscs, 

 in stagnant pools, numerous small worms with triangular heads. 

 They are furnished with suckers by means of which they firmly 

 adhere to the bodies of the molluscs ; soon they lose the tail, which 

 before had served for locomotion, and become surrounded with a 

 mucous substance, or in short become encysted. While in this 

 state it becomes transformed into an intestinal worm — the Distoma. 

 This however will not be developed fully unless it pass into the 

 intestine of some animal suited to its nature. This may easily 

 happen by the mollusc being swallowed by the sheep, the ox, or 

 even by man — in either case it will speedily be digested, and thus 

 the encysted worm will be liberated, and in due time will make its 

 way to its proper position, the liver, for example, in the sheep, and 

 is then called Distoma hepaticum, or in common language, the 

 fluke, whose pestilential effects are but too well known. In the 

 liver of their host, these Distomata, or flukes, become sexually 

 mature, and produce immense numbers of eggs, which pass into 

 the intestines and thence with the feces they escape and fall upon 

 the soil. These eggs firmly protected by their hard shell, may be 

 washed by the rain into stagnant pools, where they may soon be 



