92 DEVELOPMENT OF 



croscope, it will be seen that their transverse section is 

 perfectly circular or very slightly depressed, and that their 

 integument, and, it might almost be said, their whole 

 substance, is composed of a mass of regular, nearly equal- 

 sized, closely packed, globular cells or vesicles, (gland- 

 follicles ?) ; and that this substance incloses an internal 

 cavity of nearly the same shape as the body externally. 

 In this cavity a series of globular germs is distinguish- 

 able, and as the body in which they are contained 

 which though motionless is yet living, increases in size, 

 until it is more than a line in length, these germs also 

 increase and become active cercaria-\ike creatm^es, that is 

 to say animalcules with a disto?na-like body, and a long 

 moveable tail ; but the tail in this instance is both thick 

 and clavate, and the fully-developed animal is the Dis- 

 toma dujplicatmn of Baer.* 



The course of development of these Cercarim within 

 the parent receptacle, corresponds entirely with that of 

 the true Cercarice, and after quitting their '''■nurses'' it 

 would appear that they necessarily undergo also the same 

 change as those, as they shortly afterwards cast off the 

 thick tail, and move about as Bistomata ; but it has hap- 

 pened to me as it did to Baer, that they died before 

 becoming pupcB, notwithstanding which, however, I have 

 no doubt at all that they do enter into that state, but 

 probably only under pecuharly favorable circumstances. 

 As multitudes of these Cercarics occur in many muscles 

 within their sacculi, and also occasionally out of them, 

 and as they are sometimes present in some, and even in 

 many muscles by many thousands, it might be expected 

 that they would be met with plentifully in the form which 

 they finally assume. This is not the case however, and 

 although Baer and several others have examined the 

 muscles with great care in order to find entozoa, no true 

 distoma has been discovered in them ; we are made ac- 



* Baer's meuioii- upon Distoma duplicatum will be found in tlie series of 

 his other memoirs upon the Oercarm and other eutozoa, inchidiug Flauaria, 

 in ' Nov. Act. Acad. Leop. Car. Natur. Curios.' xiii, tab. 28. 



