380 MOLLUSCA THEIR PARASITES. 



Having collected some snails in a wet season no mites could 

 be discovered on them ; but placing them in jars to exclude 

 the moisture, their acaridan pests were seen upon them after 

 the lapse of some time, varying from five or six days to 

 three weeks. Reaumur has counted upwards of twenty 

 mites on the same snail ; and he tells us they are rarely seen 

 at rest, but are almost always creeping about, which they 

 do with extreme quickness. They are usually noticed near 

 the collar of the snail on the exterior of the body, but 

 Reaumur believes they are there by accident, and that their 

 natural place is within the intestine. The mites, he says, 

 are continually on the watch to enter the vent whenever the 

 snail has occasion to open this aperture ; and it is no sooner 

 opened than they rush in and walk quickly up the canal. 

 The reason that we find them on the surface is this, they 

 are pushed out of the intestine along with the excrements, 

 and they must remain on the surface until a favourable 

 opportunity presents itself for their re-entrance. In Cyclo- 

 stoma elegans, Reaumur found acari in the very middle of 

 the intestinal canal.* 



The Rev. Leonard Jenyns has found the same mite (Phi- 

 lodromus limacum) on some of our English slugs (Limax 

 variegatus and Arion empiricorum) ; and his observations 

 respecting it agree generally with those of Reaumur. But 

 he differs from the great French naturalist in believing that 

 the natural habitat of the mite is the pulmonary cavity, 

 and not the intestine. " I am inclined to think," he writes, 

 " that this cavity is its principal residence, whence it only 

 comes forth occasionally to ramble upon the surface of the 

 body. In one instance, I confined in a close box a slug 

 which, to all appearance, was free from parasites. On open- 

 ing the box a day or two afterwards, I observed very many 

 crawling about the slug externally, all of which would seem 

 to have proceeded from the pulmonary cavity. On another 

 occasion I observed these insects running in and out of this 

 cavity at pleasure ; and some which I saw retire into it never 

 re-appeared, although I watched the slug narrowly for a 

 considerable time. It is remarkable, as Dr. Shaw observes, 

 that the slug appears to suffer no particular inconvenience 

 from these parasites, and even allows them to run in and out 

 of the lateral orifice without betraying the slightest symp- 

 toms of irritation." f In England the mite seems to have 

 been found on slugs only; in France it infests slugs and 



* Hist, de 1'Acad. Roy. des Sc. for 1710, p. 414. 

 t London's Mag. Nat. Hist. iv. 539. 



