PARASITES. 381 



snails indifferently ;* but in Scotland I have never seen it 

 on either tribe — enjoying there a joke-provoking immunity. 

 — Two or three different mites are born to plague even the 

 fresh-water mussels, by crawling with their spinous feet over 

 the cloak of the animal, or hiding between the layers of its 

 delicate gills. Hydrachna concharum of M. Baer, which 

 may be identical with the Limnocharis anodontge of Pfeiffer, 

 and the Trombidium notatum of Rathke, thus fret the Unio 

 pictorum and the Anodontas.f These mites have not yet 

 been discovered in this country. 



The lacustrine Gasteropods are greatly vexed with the 

 parasitism of a worm that has been loosely referred to the 

 genus Gordius (G. inquilinus, Mull.), and which Drapar- 

 naud erroneously identifies with the Nais vermicularis. — 

 Midler says that the worm resembles in every respect the 

 tentaculum of a Planorbis.J It attaches itself to any ex- 

 posed part of the skin, but prefers a station between the 

 collar and the neck of the snail, where several may be no- 

 ticed with one end fixed in the flesh, while the other is ever 

 moving with a sort of painful twisting and twining motion — 

 painful because it reminds one of the worm that never clieth. 

 The ill they occasion is of uncertain nature ; and probably 

 there are more than one species, for it is not likely that 

 the parasite of the fluviatile Mollusca of Europe is identi- 

 cal with that of those of the New World. After describing 

 the Physa heterostropha of the United States, Dr. Gould 

 says : " On looking carefully about the neck of the animal 

 of this shell, we find him beset with numerous little things 

 looking like short, minute, white lines, which are, in truth, 

 little parasites (Gordius inquilinus, Mull.), attached like 

 leeches, and which derive their nourishment from the fluids 

 of the animal, without his having the power to dislodge 

 them."§ 



The leech-like form of this parasite reminds me of another 

 which you may occasionally find in the shells of Bivalves, 

 lurking between the branchial leaflets. This is the Hirudo 

 grossa of Miiller, who found it in the Artemis exoleta. I 

 have found it frequently in Cyprina islandica, and once in 

 Cardium echinatum ; and although I could not detect a con- 

 sumption in the fish, yet from the character and relationship 



* Loudon's Mag. Nat. Hist. v. 697. 



+ Bull, des Sc. Nat. Fev. 1829, p. 295. 



X Verm. Terr, et Fluv. Hist. ii. 33. — Draparnaud mistook the worms for 

 a kind of trachea, whose function it was to separate the air from the sur- 

 rounding water. Hist, des Mollusques, 49. 



§ Invert, of Massachusetts, 213. 



