DISTOMA HEPATICUM. 269 



which are furnished with cilia, but at the same time also with a 

 mouth and the commencement of a ventral sucker, we may, just as 

 in the similar embryos of Aspidoyasta conchicola described by 

 Aubert, find an example of the form above described, which, like 

 the tailless young Distoma enclosed in the nurse, pass directly 

 and without any active migration into the mature form, when 

 they are introduced into other animals. We must not, however, 

 allow ourselves to be misled into regarding embryos formed in 

 this way as being always young Distoma which become converted 

 directly into the mature form ; they may also be nothing but the 

 earliest stages of the Redice, and this circumstance must be well 

 borne in mind. That D. hepaticum bears a brood of this kind 

 certainly does not appear to me impossible, according to the 

 observations hitherto made, but it is very improbable, and, in 

 accordance with the results given above, I regard it as most 

 probable that the young brood of D. hepaticum possess a tail. 



When De la Valette set about administering the tailed, free- 

 living forms, that is to say the Cercarice, the result of a meta- 

 morphosis of these forms into mature Distoma did not occur. 

 He then directed his attention to the forms originating from the 

 Cercarice just referred to, which are enclosed in cysts, and although 

 still asexual, are already in other respects somewhat further 

 developed. Such enclosed young Trematoda are found partly in 

 the bodies of mollusca and aquatic insects, and perhaps also — 

 if we may draw conclusions, per analogiam, from the Monostoma 

 to the brood of Distoma — partly free in the water. At least, De 

 la Valette saw, under his own eyes, young Monostoma free in 

 the water surround themselves with a cyst, certainly by the 

 solidification of a fluid secreted by the Distomum. 



The further development of such enclosed Cercarice, when they 

 have reached the stomach of an animal, takes place in the follow- 

 ing way : The Cercaria, which has become converted into a 

 young Distomum, escapes from the cyst with either the head or 

 the tail foremost. The nucleated cells of the Cercaria have 

 already disappeared ; in the armed species the hooks are seen 

 distinctly, and in all we see particular systems, such as the 

 system of excretory vessels and the ciliary vessels with thin 

 hyaline globules. The contractile sac of the excretory system 

 is elongated ; the foundations of the testes and germ-stock make 

 their appearance. When administered in this state, the young 

 Distoma are quickly provided with germ-stock, testes, and ovaries ; 



