84 DEVELOPMENT OF 



species with tlie Cercarice inclosed within them, or with 

 the Bistoma into which the Cercarice are transformed after 

 passing through the pupa state. They are thus indivi- 

 duals which remain stationary at a particular stage of 

 development, not much higher than that at which the 

 Cercarice commence, and which have undergone as it were 

 a retrograde metamorphosis, in order to promote the 

 perfect development of the brood of Cercarice committed 

 to them ; in other words, they are individuals of the kind 

 which I have in the foregoing pages denominated " nurses J" 

 The existence of such " nursing" animals within a species 

 can no longer be considered singular, when the facts ad- 

 duced in the preceding sections are brought into compari- 

 son ; and their origin can also no longer appear mysterious 

 when it is remembered, that sluggish creatures resembling 

 the " nurses' or "parent nurses" of the Cercaria echinata 

 arise from the metamorphosis of the active young of Mo- 

 nostoma mutabile. In the following pages I wül relate 

 precise observations, to show that these saccular and ap- 

 parently altogether lifeless " nurses' originate directly 

 from very lively animalcules; but before proceeding to 

 these observations, I mil offer some considerations on the 

 subject of the " nursing" animals now before us. 



In the earlier researches upon the Cercarice, the sup- 

 position is sometimes directly broached and sometimes 

 understood, that the " nursing' animals, the supposed 

 saccuh, were probably only the smaller and distended 

 bodies of the Cercarice, from which, consequently, they 

 must have originated through a kind of metamorphosis ; 

 but I believe that this opinion is confuted by what has 

 been stated in the preceding observations, and that there 

 is no doubt but that the " nurses' are developed from 

 very minute oval corpuscles or germs, and that thus they 

 and the Cercarice contained within them, pass through a 

 parallel series of developments, and that there is no room 

 for supposing that any metamorphosis or transformation 

 occurs from one series to the other. 



It seems as if the earlier observers could not under- 



