352 THE INFLUENCE OF LIVING SURROUNDINGS. 



the intestinal cavity of the Holothuria}. These have recently 

 been described by Kossraann under the name of Lecanurius ; 

 but these small creatures have not assumed the organisation 

 of true and degenerate endo-parasites, but possess all those 

 organs which are found in free-living species or in ecto-parasites 

 and which enable them to change their dwelling-place rapidly 

 and at will. 120 



On the other hand, I found on the skin of the very same 

 species of Holothuria which harbours in its intestine the crea- 

 tures just described, a Eulima which is far more degraded in 

 structure than any other species of the genus. The front of the 

 head which bears the mouth is drawn out into an extremely 

 long proboscis, which pierces quite through the very thick skin 

 of the Holothuria, and the mollusc is just as securely anchored 

 by it as is the Pachybdella by its style or holdfast. But this 

 proboscis must also act as a food-sucker, since it bears, at the 

 end it inserts into its host, a simple mouth without any gnawing 

 apparatus. The foot, which in other species living on the skin is 

 well developed, has here wholly disappeared (see fig. 95), and 

 eyes are likewise wanting. Thus we perceive that the effects 

 xisually produced by the condition of living in the intestine, in 

 this instance have not been able to impress the character of endo- 

 parasites on these living in the Holothuria ; while, on the other 

 hand, a true ecto-parasite has been modified in the way com- 

 mon to endo-parasites, although it belongs to a group of animals 

 of which the numerous species live, without exception, on the 

 skin of Echinodermata, but nevertheless are thereby modified 

 to so insignificant an extent that their parasitic nature has even 

 been altogether denied. 



The causes which have so far come under our consideration 

 as lying within the agency of living organisms and occasioning 

 modifications in animal forms, sink altogether into the back- 

 ground as compared with one now to be discussed, and about 

 which much has been written, and not a little that is false ; 

 namely, Hybridisation. This word signifies the fertile union of 

 two individuals which according to our systematic classification 

 are supposed to belong to two different species, and which are 

 supposed, by a certain school of naturalists, never to have been 



