210 ENTO-PAEASITES AND HYGIENE. 



If this cannot be proved without doubt, we have before 

 us a so-called pseudo-parasitism, cases of which are by- 

 no means rare, often puzzling and taxing the skill of the 

 scientist as to their true nature, because they may grad- 

 ually pass into true parasitism. 



We may observe, for instance, that one animal is 

 found usually and regularly in the interior of 

 another, thus arousing the suspicion of being a true para- 

 site. On closer examination, however, we find that such 

 supposed parasites do not get their food from the ani- 

 mal they inhabit ; they only take part of the meals of 

 the host by grasping what they can of the food of the 

 latter and claim inii' it for themselves. Such animals are 

 called, in opposition to the true parasite, table compan- 

 ions or ^'■commensals.'''' 



A very nice case of such commensalism is that of certain 

 mollusks (Cyprina islandica and Arga arenaria). They 

 house in their mantle cavity a worm (Malacobdella), which 

 might easily be taken for a real parasite, if close obser- 

 vation, especially the examination of the contents of its 

 intestines, did not prove the contrary. Never are blood 

 or tissues of the mollusc itself found in the worm, but 

 mostly little animals and plants that float in the water, 

 such as algge, diatomese, little crabs, etc. The reason 

 for this strange behavior is obvious. The "table com- 

 I)anion " or host is seated where especially favorable cir- 

 cumstances make it easy for him to get food ; the worm 

 mentioned has its abode just where a constant stream of 

 water produced by the gills of the mollusc, laden with 

 many small particles of food-material, passes by, so that 

 it need only help itself to what is floating by in order to 

 be provided with its few necessities of life. 



Beside these commensals or pseudo-parasites we have 

 in the true parasites creatures which concerning their 

 food are utterly dependent upon another animal. They 

 are not able to live on any thing but the liquid contents 



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