Development of the Infusoria. 255 



planorbes, and as I think the PL carinatus. On examining the 

 water in a few days, I observed with the naked eye some very 

 active animalcules, which I immediately recognised as cercariae 

 by their very rapid motions. Their numbers increased on the 

 following days. My observations were afterwards interrupted. 

 On closer examination the animal appeared to be the Cercaria 

 Lemna, Muller (Encyclop. Method. PI. 8. fig. 8. 12.), a species 

 which, as Nitsch supposes, is probably identical with the Cercaria 

 major (Beytr. zur Infusorienkunde, Tab. ii. fig. 1-8.), for it re- 

 sembles it exactly ; only I could detect none of the fine hairs on 

 the tail, which, however, only appeared to Nitsch with very 

 high powers, and with the field of vision half illuminated. I 

 employed a power of 48. For a long time I was quite de- 

 lighted with the singular movements of these Cercariae, but I 

 turned my attention principally to their internal structure, which 

 is not sufficiently known. 



The cercaria which I observed was about \"' in length. The 

 tail and the body were nearly of the same thickness ; the latter 

 was longitudinally striated. The substance of the animal was 

 homogeneous, transparent, and of a milk-white colour. At 

 the anterior extremity was the round extensile opening of the 

 mouth, from which it appears to be surrounded with a wreath 

 which was notched. This mouth is succeeded by a smaller 

 very distinct oesophagus, between which and the mouth the in- 

 testinal canal seemed to be somewhat narrowed. I saw no 

 trace of the forked organ noticed by both Nitsch and Baer ; nor 

 was the course of the intestinal canal here marked by any dark 

 spot, but it appeared to me merely to resemble a broad band 

 extending from the mouth to the anus. The ovaria were very 

 conspicuous, placed on the two sides of the body. When the 

 animal was moderately extended, they made several convolutions 

 in the form of an S, from the two sides of the oesophagus to the 

 cloaca. The convolutions were less distinct when the animal 

 was much extended. The hinder extremity was very short and 

 broad, where the ovaria formed two irregular masses. Their 

 texture appeared granular, and had a beautiful appearance, 

 their dark colour being contrasted with the transparency of the 

 rest of the body. Their exit into the cloaca I could not dis- 

 tinguish. Their origin towards the anterior part of the body 



