THE TEEMATODA. 71 



in ascent, and I have not been able to detect any more 

 generations ; and am, consequently, not in a condition 

 at present to trace the origin of the Bistoma further 

 back. I entertain, however, the not unfounded supposi- 

 tion, that the ''parent nurses' do not arise from other 

 similar creatm^es, but that thei/ proceed originally from 

 ova; for, as is stated above, (p. 61,) it must be assumed 

 that the full-grown fluke, like all other species of this 

 genus or family, is oviparous. I will afterwards exhibit 

 the reasons wliich induce me to beheve that it is the third 

 generation, or the "j>?ö;re^^^ nurses' which are produced 

 from the ova ; whilst I will also prove, that animals such 

 as those which I have wdonedi " nurses" and ''parent 

 nurses^' belong to the series of development of the Disto- 

 mata, and derive their origin from ova, which have been 

 produced by the Distomata themselves. 



With regard to the link which is deficient in the de- 

 velopment of the "parent nurses," and for want of which 

 we are unable to deduce their immediate origin from the 

 full-grown Bistoma, so that all the generations here 

 referred to would constitute a complete cycle {cyclus), we 

 may form a tolerably correct notion of what it is like by 

 help of the observations which were published by Siebold 

 some years since, in the first number of ' Wiegmann's 

 Ai'ch. für Natm^g.' 



Monostonium mutabile is an entozoon of the trematode 

 family, which inhabits several of the cranial cavities lined 

 with mucous membrane in certain water birds, and is one 

 of the small trematoda, the embryos of which are fully 

 developed in the ova, while still in utero. The young 

 embryo is frequently hatched before or just as the ovum 

 is expelled, and in this case, the Monostomum mutabile is 

 ovo- viviparous. The newly -hatched young (fig. 1 b) are 

 elongated, oval, furnished at the anterior extremity with 

 some short lobes, which the animal is able to protrude 

 and retract, and its whole surface is covered with vibratile 



