I'AHASITISM IN PLANTS AND ANIMALS. 287 



takes to a secleatary life, and devotes its energies solely to the 

 reproduction of the species. Examples of such a "retrograde de- 

 velopment" are numerous amongst the Crustacea, this class pos- 

 sessing a remarkable plasticity, by virtue of which their organs 

 appear to be capable of almost infinite modification so as to meet 

 the varied pressure of outward circumstances. An interesting 

 example of similar degradation in another class of animals, the 

 Acarida or mites, is seen in a little parasite which occurs in the 

 sebaceous follicles of the human nose : a follicle becomes inflamed 

 and fills with a curdy or purulent fluid, wherein is found a 

 minute, worm-like creature with eight very rudimentary legs, — 

 a character which shows it to be a degenerate mite. Another 

 parasite of the same order is that which gives rise to the com- 

 plaint called " itch." This is a f;drly-well developed and active 

 acarus, which makes small burrows beneath the skin and there 

 deposits its eggs. 



The condition called by Van Beneden, " commensalism, " was 

 referred to briefly at the outset, and now let me explain more at 

 length the nature of the association ; — parasitism it can scarcely 

 be called. Well-known illustrations may be found amongst the 

 sea-anemones and hermit-crabs. Most of you, no doubt, are 

 aware that " hermit-crabs," being very soft and ill-protected in 

 the hinder parts of the body, have the habit of ensconcing them- 

 selves in the dead and empty shells of univalve molluscs, such 

 as whelks and periwinkles, removing themselves, as they grow, 

 into larger and still larger tenements according to their need. 

 Some of these crabs are found invariably in company with anem- 

 ones, which attach themselves round about the lips of the shell 

 occupied by the crab. To British naturalists the most familiar 

 instance is that of a very pretty anemone, Adamsia palUata, 

 which is constantly found attached to shells tenanted by the 

 hermit, Pagurm Prideauxii. It is very rarely that either of the 

 two animals is found separately, but in what way they are ser- 

 viceable to each other it is not so easy to say. Possibly the 

 anemone may be a protection to the crab by virtue of its sting- 

 ing power, for it belongs to a group of animals, all of which are 

 richly provided with urticating cells similar to those which often 



