350 Dr Eschricht's Inquiries toncerning 



troublesome guests. But all such hypotheses are of very lit- 

 tle value ; and the answer to the inquiry, how intestinal worms 

 are propagated, is to be obtained only by long and laborious 

 investigations into nature, and will probably be found very 

 different in different species. 



That the inquiry will he long and laborious will not be 

 doubted, when we reflect on the history of the instances re- 

 ferred to : and the labour must needs be far greater than 

 that which is usually bestowed upon inquiries of this kind by 

 medical men. The case of the Sarcoptes may serve as an il- 

 lustration. For many years its existence was known from the 

 tales of fishermen and galley-slaves, but medical men could 

 nowhere find it. At last, a young French student had the 

 effrontery, before the French Academy, to mingle mites with 

 the humour evacuated from the pustules of the itch, and thus 

 the insect producing the itch was for about twenty years re- 

 garded as an acarus ; at length the Corsican peasants were 

 consulted, and they pointed out the way in which the real Sar- 

 coptes might be discovered. All the while this parasite was 

 as common as the itch itself, and large enough to be easily 

 detected by the naked eye ! 



That a particular inquiry will he required for each particular 

 species, may be concluded from the fact, that each species se- 

 lects generally certain animals, and in these certain regions ; 

 as, for instance, the Lemma elongata (whose anatomy has been 

 given by Dr Robert Grant) selects the eye of the Greenland 

 shark, the Coronula balcenaris the skin of whales, the Otion 

 auritum^ the Coronula, different species of Pinnotheres select 

 certain species of living bivalves, and the Paguri certain uni- 

 valvular shells. 



As a most curious instance of this predilection of parasites 

 for certain localities, we may mention the parasites which re- 

 gularly, in winter, fill a particular sac connected with the tes- 

 tis of the Cephalopoda. These parasites have lately been in- 

 troduced by Dr Carus into the parasite fauna under the name 

 of Needhamia. (His memoir will be published in the next 

 volume of the Acta Leopoldino- Carolina.) Swammerdam fur- 

 nished the first description of them ; Needham, in his micro- 

 scopical observations, nearly a century ago, supplied a good 



