ABSOEPTION OF FOREIGN ELEMENTS. 



75 



simple vegetables known as Lichens, which, however,, are still 

 generally classed as a distinct group of plants. It may at 

 first sight seem somewhat bold to assume that living plants, 

 even of the simplest conceivable structure, could constantly, 

 or almost constantly, be so associated with an animal as to 

 seem one of its histological elements. But this hypothesis 

 assumes a high degree of probability when we remember that 

 numerous parasites occur with unfailing regularity in certain 

 organs of every individual, or nearly every individual, of a 





Fig. 19. — Longitudinal section of Sphenrpus iStcenstrtipii S. The skin of the creature, 

 ep, which is thinnest above, has agglomerated grains of sand throughout its sub- 

 stance. 



species — for instance, the larvre of certain Nematodes in the foot 

 of the common snail ; when, moreover, we take into considera- 

 tion that different animals, more particularly Sponges and 

 Polyps, frequently take up dead or living foreign bodies and 

 utilise them as normal elements of the tissues (fig. 19). 



Of course no decisive answer can be arrived at by this 

 method, and only experiment can find one. But it seemed to me 

 to be advisable to state both these possible solutions, and to 

 bring forward those facts which may perhaps soon require us, if 



