THE TREMATOD A. 73 



Siebold, and it might at first sight be taken for one of 

 the polygastric infusoria of Ehrenberg, which also move 

 by means of cilia r whilst in the next form which it 

 assumes, the young Monostomum bears an undeniable 

 resemblance to those animals which I have termed 

 " Miü'ses" and "parent nurses," in that species of the Tre- 

 matoda Avhich is developed from the Cercaria ecJiinata. In 

 this second or altered form (the necessary parasite) the 

 animal, according to Siebold's figure, appears to have a 

 mouth at the anterior extremity, within which there seems 

 to be a muscular oral cavity. At a short distance below 

 the mouth there is an evident depression, indicating as it 

 were a short neck (fig. 1 d,) resembling that in the 

 " nurses' and ''parent nurses " at the posterior part of 

 the body there are two lateral processes, and behind these 

 a shorter, moderately thick tail or prolongation of the 

 body, which in like manner correspond to the same parts 

 in those creatures. 



If objection be made to the view which does not regard 

 the change from the active state represented in fig. 1 b, 

 into the sluggish animalcule fig. 1 c, as a metamorphosis, 

 I vrill with respect to this point further remark, that the 

 mode in which these two apparently different animals lie 

 one within the other, recalls completely the conditions 

 under which the sluggish short-tailed Brachyiirus lies 

 within the active long-tailed Macrourus, and the form 

 which it has on escaping from the ovum, but which it 

 afterwards loses. f 



Now, since the progeny of the Monostomata — a family 



* I beKeve also that it may be regarded like the other Tolygastrica, as 

 possessing many stomachs ; and it appears to me, that the dispute about the 

 many stomachs of the Polygastrica may carry us further than to assertions 

 which are merely opposed to each other, if the analogy or want of analogy 

 of these infusoria, with the earliest foetal conditions of several forms of 

 animals, is followed out. 



f An abridgment of my observations on the metamorphosis of the Crus- 

 tacea {sand-crab. Hi/as araneus ; and the hermit-crab, Fagtirus Bernhardus ;) 

 and on the various regions in the water inhabited by the young ones in the 

 various stages of their development, is given ia Oversigt overdet kongl- 

 danske Videnskabsselskabs; Porhandliager for Aaret, 1840 ; and in Valentin's 

 Repert. 1840. 



