92 THE ORIGIN OF PARASITES. 



but ultimately becomes parasitic, 1 at the same time losing not only 

 its shell which is also the case with certain other snails but also its 

 locomotor, sensory, and alimentary organs, and degenerates into a 

 simple sac filled with sexual products. In the form of this " snail- 

 sac " the parasite is found in the body-cavity of the vermiform Holo- 

 thurian (Synapta digitatct), having its thickened knob-like anterior ex- 

 tremity inserted into the intestinal vessel of its host, so that it may 

 easily be mistaken for a true organ of the latter. No one, without 

 knowledge of the young form, could recognise its Molluscan nature. 



If we regard this retrograde development as a consequence of para- 

 sitism, we do not thereby mean to imply that this exerts its influence 

 from the commencement, and with full force in each individual animal, 

 and that the same process is repeated de novo each time in a similar 

 manner. The influence which the external relations of life exert upon 

 the development of an organism in the present case, as everywhere 

 else, can only have been a gradual one, which must have continued 

 to work for many generations before it could produce such extreme 

 effects. It is not a sudden transformation, but a slow and steady 

 progressive adaptation to the conditions of a parasitic mode of life, 

 of which we see the results in the above-cited organism. We must 

 accept the conclusion that the Mollusc to continue with our example 

 has not exhibited this particular form of parasitism from the com- 

 mencement, but has only gradually adopted the above-described mode 

 of life. 



When the number of parasites in any group of animals is in- 

 creasing, we often see also the various stages of parasitism in exist- 

 ing forms allied to each other. The sum of the degeneration and 

 transformation is then seen to be of different extent in different 

 species, for the transformation of the organism in no case goes further 

 than the circumstances of the parasitic life require. Step by step we 

 can see how, under such circumstances, animals that feed usually on 

 organic detritus, like the Asellidtv, or lead a predatory life like the 

 free-living Copepoda (represented in our waters by the genus Cyclops) * 

 exchange their free life for a parasitic one. Often they are only tem- 

 porary parasites, differing from the most nearly related forms perhaps 

 only in the possession of more powerful hooks, whilst in other cases 

 they continue for a longer period upon their host. * They lose the 

 power of locomotion they previously possessed, since their extremities 

 atrophy in consequence of disuse, and become stunted in their growth 



1 I have followed in the account given above the usually accepted view, but I may 

 add that the transformation of the snail into the so-called snail-sack, has not as yet been 

 directly observed. 



2 See v. Nordmann, Mikroyraphitche JBeitrdge, Bd. ii. : Berlin, 1832. 



