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 Sresiop saw in Monostoma mutabile 
a parasite lodged which agreed in form with that of the germ-pouch 
of Cercaria 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 Sresoip has given a probable answer to 
this enigma. He very commonly found in the aquatic larve of 
Liphemeride, Phryganide, Libellulide, &c., as well as in the perfect 
insects, encysted Cercarie 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!.] 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 Meduse, 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 Sresotp Die Band u. Blasen-wiirmer, Leipsig, 8vo. 1854, pp. 2I—3I. 
