16 



vibratile caudal appendage: they escape from the parent 

 cyst, bore their way out of the snail, and disperse them- 

 selves as free swimming ciliated Cercariform animal- 

 cules in the water. After a brief enjoyment of this free 

 and active state of existence, they shrink in size, the vi- 

 bratile tail is cast off, and they attach themselves to the 

 skin of the snail. Here they become buried, form for 

 themselves a pupa-case out of the condensed mucus, and 

 are metamorphosed into true Distomata, which gain their 

 parasitic habitat by piercing the soft integument of the 

 water-snail. Thus we have a Trematode entozoon succes- 

 sively assuming the form of a Gregarina, a Cercaria, and 

 a Distoma ; one impregnated ovum developing many in- 

 dividuals of the second form, each of which finally passes 

 into the third form*. 



But this does not give the entire metagenetic cycle of 

 the Entozoon. The gregariniform parent of the cercarial 

 larvae is most probably the product of the ovum of the 

 Distoma tarda, not the ovum itself, the development of 

 which, in this species, has not yet been seen. From the 

 analogy of those Trematoda in which that development has 

 been traced, e. y. Monostoma mutabile, Distoma tereticolle, 

 Dist. hians-f, Dist. hepaticum, Dist. cylindraceum, Dist. 

 globiporum%) Dist. duplicatum, Dist. longicolle and Amphi- 

 stoma subclavata, the first product of the ovum is a free- 

 swimming -ciliated animalcule, and so like a polygastric 

 infusory as to have been described as a Param&cium. Sie- 

 bold, to whom we owe the most precise and valuable in- 

 formation of this first link in the chain of self-active indi- 

 viduals, found that this animalcule was hatched in the 

 oviduct of the Monostoma mutabile, was excluded from the 

 egg by pushing off its opercular cap, and then escaped 

 from the vulva. The ciliated integument inclosed a fluid 



* Steenstrup, op. cit. p. 52, pi. 3. Siebold, Archiv fiir Naturge- 

 schichte, 1843, p. 321. 



f Mehlis, Oken's Isis, 1831, p. 190. 



| Nordmann, M ikrographische Beitrage, Heft 2. p. 139. 



