232 Transactions of the [ToumZiSTwo*' 



III. — Cereariw, 'parasitic on Lymnsea Stagnalis. 



By Jabez Hogg, Hon. Sec. E.M.S., &c. 



{Read before the Royal Microscopical Society, April 13, 1870.) 



Plate L. (upper half). 



The superior means, and increased facilities, for microscopic in- 

 vestigation whicli we now possess, over observers of a former 

 generation, offer some temptation to young and enthusiastic workers 

 to go over ground apparently exhausted, or at least well worked. 

 This is often useful, if for no other purpose than that of comparing 

 notes, weeding out supposed new species, rooting out old theories, 

 and perchance clearing uj) some obscure or doubtful point in the 

 history of an animal or vegetable. It will be readily conceded that 

 owing to the imperfections of the instrument employed up to within 

 the last quarter of a century, and the method of preparing and 

 mounting objects, many inaccuracies have crept into our scientific 

 descriptions, and have been accepted for no better reason than because 

 presented under the authority of some well-known name. 



Such thoughts were passing through my mind as I was leisurely 

 observing the movements of some water-snails near the edge of the 

 lake in the Botanical Gardens towards the end of the autumn of 

 last year. I stooped to pick up a fine specimen of Lymnxus stagnalis, 

 and observed suspended from its pulmonary cavity a mass of minute 

 thread-like bodies, which my pocket lens enabled me to dete^ mine 

 were of a parasitic nature. I put the specimen in my packet, for 

 more careful examination at home. Before depositing the snail in 

 my aquarium, I gently detached a few of the little animals, and put 

 them into a shallow cell. On viewing them with a half-inch objec- 

 tive I was arrested by their wonderful activity ; they plunged in 

 the water and lashed it about in their wild attempts to escape. 

 Finding all efforts to do so impossible, they violently tore their 

 tails from their bodies, swam about for a few moments, even more 

 vigorously than before, and at length growing weaker, died appa- 

 rently from exhaustion. I had no doubt that my specimens were 

 the larvae of a trematode worm ; but as I had not previously seen 

 any exactly like these, which differed in some particulars from those 

 figured by various authors, and was so very different from one given 

 in my paper " On the Water-snail," * I was rather inclined to beheve I 



EXPLANATION OF PLATE L. (upijer half). 

 Fig. 1. — Ccrcaria furcata, seen when first removed frona the Lynmajus. 

 „ 2. — A profile, or side-view, of animal at perfect rest, and suddenly killed. 

 „ 3. — Seen in the act of breaking or dividing into two portions. 



* ' Trans.,' vol. ii., p. 102, 1854. 



