G. GENERAL NATURAL HISTORY AND ZOOLOGY. 273 



the bony fishes, constitute a nest of worms, etc., lodging a 

 living population which is never seen elsewhere. These are 

 all characterized by their different peculiarities, many of them 

 occupying internal cavities in which they never see the light. 

 These parasites are also remarkably constant to particular 

 species of fish, Professor Van Beneden stating that usually, 

 wherever found, the same fish will have the same parasites, 

 the latter very often playing an important part in the iden- 

 tification of the species. Among the species examined, the 

 turbot was perhaps the most thickly crowded with intestinal 

 worms, while Atherina presbyter was absolutely the only one 

 in which such parasites did not occur. A corresponding spe- 

 cies very common on our own coast (the A. notata), known 

 as the fryer or sand-smelt, probably shares in this peculiarity. 



In further continuation of his subject, Professor Van Bene- 

 den remarks that worms and crustaceans found living upon 

 the skin of fish are not all to be considered as parasites, since 

 this involves the living of the one at the expense of the oth- 

 er, and many forms merely ask of their neighbor a place of 

 refuge and defense, without taxing him in any way for sup- 

 port. Animals of this kind associating in common, each hav- 

 ing its independent condition without preying upon or deriv- 

 ing food in any way from the other, are called commensals 

 a term which signifies their feeding at a common board, and 

 not upon each other. These commensals may be divided 

 into various groups. Thus some of them are tied, while 

 young, to a good neighbor, who lets them go when they 

 have been towed to their destination. Others are adherent 

 at all periods of their lives, but can let themselves go at will, 

 exercising their own discretion in selecting the place of at- 

 tachment to the body of their neighbor, as in the remora, or 

 sucker-fish. Others, again, have freedom of choice while 

 young, and at a certain period attach themselves permanent- 

 ly for the rest of their lives. Their lot is then connected 

 with that of the host they have chosen. This is the case 

 with some of the barnacles, etc. 



Other commensals, again, are never fixed, but take up their 

 position near a neighbor, and never leave him. They remain 

 in the digestive tube, at one end or the other, or they place 

 themselves under the mantle of their acolyte, and make oc- 

 casional sorties at favorable moments, as in the common oys* 



M2 



