350 THE INFLUENCE OF LIVING SUKRODNDINGS. 



tion of these ■will once more set some of tlie general principles 1 

 have already laid down in a clear light. 



If desuetude were invariably to be regarded as a primary 

 cause of the disappearance of organs no longer in exercise, it 

 would be very difficult to understand why, tinder apparently 

 identical circumstances, identical results should not follow, i.e. 

 the disappearance of an organ. All the free- swimming larvse of 

 the lower Crustacea have similar swimming organs, namely legs, 

 and all alike are thrown out of use by the settlement and 

 attachment of the parasite. In spite of this, the legs are by no 

 means universally absorbed in the same way. In some species 

 one disappears fii-st, in other species another ; sometimes too a 

 few limbs are spared and remain attached to the body, though 

 perfectly useless. Hence the same cause affects the same organs 

 very diiferently in different species ; and this proves that the 

 absence of a disused organ is not a mere mechanical result of 

 desuetude, but, on the contrary, is subject to other determining 

 influences according to the peculiarities of the animal whose 

 organs of motion are no longer exercised. We arrived at the 

 same conclusion in a former section when considering the inani- 

 mate conditions of existence, and I will endeavour in this 

 chapter to illustrate this point more f ally by a few other striking 

 instances. 



As a rule a tolerably sharp distinction is made between ecto- 

 and endo-parasites ; the former being such as live on the outer 

 skin of animals, e.g. the louse, the latter living in the interior 

 organs. It is also regarded as an almost universal rule that 

 ecto-pai'asites are of less degraded forms than endo-parasites ; 

 however, there are some very striking exceptions to this rule. 

 The most remarkable exceptions known to me are the following, 

 which I myself observed in the Philippine Islands. 



Holothurians, like all animals, are infested by a great num- 

 ber of various parasites. Besides the Fierasfer (a fish) and 

 Piniiotheres (see p. 80) which live in the Avater-lungs, other 

 parasites, molluscs, and worms are found on and in them. 

 Among the foimer Eulimu occurs very frequently on the skin 

 of the Holothurife (as also on that of Star-fishes) ; it exactly 

 resembles other imivalve moUusca, and its parasitic mode of life 



