172 CLASS V. 



that proceed from eggs of Distomes, move rapidly by means of 

 cilia with which their body is covered like that of many Infusories. 

 In such a young animal VON SlEBLOD saw in Monostoma mutabile 

 a parasite lodged which agreed in form with that of the germ-pouch 

 of Gercaria echinata. [He concludes that the germ-pouch is thus 

 shewn to be the descendant of a Trematode. The Monostoma muta- 

 bile is a parasite of water-birds and lives in cavities of their body 

 which communicate with the external world. If a Monostoma mut. 

 has produced an embryo, this can readily escape from the abode of 

 its parent into the water, and may by means of its cilia find in- 

 stinctively the animal suited to supply a fit residence for the further 

 development of the germ-sac which it contains. It may pass into 

 the interior of that animal by some one of the natural openings. 

 Having now fulfilled its office of a living envelope to the germ- 

 pouch, the ciliated embryo will die, and the germ-pouch seeks by 

 perforation that situation within the body of its host which is suited 

 to its further growth, and to the supply of due nutriment for the 

 Cercaria-brood which it contains. The greater number of suctorial 

 worms are, when they have gained their sexual organs, parasites of 

 the higher vertebrates. How then can Cercaria effect an entrance 

 into the body of such vertebrates as never come near the waters in 

 which they live? VON SIEBOLD has given a probable answer to 

 this enigma. He very commonly found in the aquatic larvse of 

 EpJiemeridce, PhryganidcB, Libellulidce, &c., as well as in the perfect 

 insects, encysted Cercarice which had divested themselves of their 

 tail, and which in fact were larval trematodes. In order to effect a 

 passive migration into the vertebrate in which their development is 

 to be completed, they must wait until their temporary host is 

 swallowed by some insectivorous land bird or mammal 1 .] Though 

 there still remains an ample field for conjecture and fancy, thus 

 much may be concluded from the observations hitherto made, that 

 in these animals (as in Medusae, vid. pp. 100, 101) a succession of 

 alternate generations occurs : that the first series does not resemble 

 the parent, but that from it young ones proceed which return to the 

 original form. The germ-pouches constitute the first series, the 

 nurses (nutrices) / they are the starting-point of animals to be born as 



1 Comp. VON SIEBOLD Die Band u. Blascn-iciirmcr, Leipsig, 8vo. 1854, pp. 21- 



