THE TREMATOD A. 75 



each stroke form the same 88 or SS figm-es ; but the body 

 and neck midulate alternately while the animal is swim- 

 ming. On fixed objects in the water the creature's 

 motions are more leech-like, in which case it makes use 

 of the abdominal acetabulum ; I have seen hundreds of 

 the Cercarice creeping in this way on the slimy cuticle of 

 Limnceus stagnalis and Planorbis corneiis, whilst thou- 

 sands were swarming about every snail in the glasses. 



Their incredible numbers frequently made the water 

 tm'bid, and I have not unfrequently observed the same 

 thing to occm' naturally in stagnant pools in the neigh- 

 bom'hood of Copenhagen and Sörö (in the marsh Flo^n- 

 meu,) in which these animalcules were swarming. This 

 observation, in conjunction with another, that they 

 always keep as it were in whole swarms around the 

 snaus, appears to me to be not unimportant with regard 

 to the mode in which they are conveyed from one animal 

 to another. 



Of the internal organs, which are visible thi'ough the 

 more than half-transparent integument, the most striking 

 is the minute, linear-pointed, hard spiculum, placed 

 above the oral cavity, immediately under the skin, and 

 which projects so much with its pointed extremity from 

 the skin, that the species has been rightly named armata; 

 besides this, there is an elongated organ which com- 

 mences at the anterior extremity of the animal on each 

 side of the spiculum, and passes down along the sides of 

 the body ; but to the middle of which I have been only 

 occasionally able to trace it, and at that point a clear spot, 

 surrounded with one or two darker rings, indicates the 

 situation of the abdominal acetabulum and its borders. 

 Before the root of the tail, as in the former species, is 

 situated a clear spot, capable of being expanded and con- 

 tracted, which from long examination of several species of 

 Cercarice, I believe to be nothing but the transverse 

 section of the tail where it is attached to the trunk, seen 

 through the skin. The body is slightly excavated poste- 

 riorly, to receive the tail, and the lateral portions which 



