THE TREMATODA. 99 



unless they also reach the organ or situation, which af- 

 fords the requisite conditions for their future development. 

 There is no doubt at all from what precedes, that another 

 animalcule described and figured by Nordmann is also a 

 pupa, viz., the JDistoma ammligerum, (tab. i, figs. 5, 10;) 

 it also inhabits the eye of the perch, but Nordmann has 

 invariably found it only in the pupa state. 



Having thus, as I hope, shown in what way trematode 

 animalcules also enter fish from without, and penetrate 

 into their organs, and immediately as it seems whilst 

 undergoing a metamorphosis, I will now only add an 

 observation which renders it probable, that the Trematoda 

 of the higher animals, at least of the BatfacJdmis, likewise 

 make their entrance into them from without ; I have, for 

 instance, found immediately under the skin of the common 

 frog {Rana temporarid) small cysts, which from my pre- 

 vious knowledge of the transformation of the entozoa, I 

 necessarily considered as 2mp(S, and the correctness of 

 this supposition was established when I opened them ; 

 for they invariably contained a trematode animal, viz., 

 an Ämpliistomum, and since similar entozoa are rather 

 frequently met with in the rectum {A. clavatum,) and I 

 usually observed ikepupcs on the mesentery, and even in 

 the cellular tissue surrounding the intestinal canal, I was 

 obliged to conclude that the individuals within the intes- 

 tine originated in those ptipce, and had entered through 

 the integument. It will not, however, be long before 

 observation will prove it to be a universal fact, that the 

 greatest part of these parasites penetrate through the skin 

 by the shortest route into the internal organs. 



WTien a metamorphosis occurs so extensively, or is even 

 universal in one division of the entozoa, the question 

 natiurally arises, whether with respect to the other divi- 

 sions of that class, it is probable that a similar trans- 

 formation is effected by a single metamorphosis, or 



