(ECOLOGY. 2O5 



through embryology was the unnaturalness of this group- 

 ing recognized. Entoparasitism therefore is one of the 

 most beautiful examples for elucidating the character of 

 coii-i'tT^cnt breeding, i.e., how animals of different system- 

 atic position acquire, under similar conditions of life, a great 

 similarity of structure and appearance. 



Symbiosis. Less frequent than parasitism is sym- 

 biosis, or the cohabitation of animals for reciprocal ad- 

 vantages. Among animals which form states, it is not 

 infrequently observed that they not only hold certain 

 animals in bondage, but even seek to protect and serve 

 them ; as, for example, in the company of ants are found 

 certain blind beetles, like Clavigcr, or some species of plant- 

 lice, or even ants of other species and genera. But such 

 cases of cohabitation correspond rather to the domestication 

 of animals, or to slavery, as carried on by man. The ants 

 hold the plant-lice in order to lick the sweet juice which is 

 secreted in their honey-tubes ; they steal the pupae of other 

 ants and rear them, to use them later as slaves. This 

 state of things rests, consequently, not upon equal rights, 

 since the one animal, in the present example the ant, brings 

 about the cohabitation, while the other animal is passively 

 led into it. 



An Instance of True Symbiosis. An instance of most 

 complete equal rights and true symbiosis is furnished us, 

 however, by a crab and an actinian, Pagnrus Pridcau.vi and 

 Adamsia palliata. Like every species of Pagnrus this 

 hermit-crab also inhabits a snail's shell, from the opening 

 of which only his legs and pincers are protruded. Upon 

 the snail-shell a small actinian fixes itself, surrounding with 

 its body the entrance of the snail-shell. \Yhen the crab in 

 the course of his growth is compelled to seek a new and 

 larger snail-shell, he invariably takes the actinian with him. 

 The advantage which the actinian derives from this sym- 

 biosis is clear; it gains a share of the booty which the 

 nimble-footed crab obtains. It is less clear why the crab 

 lays so much stress upon this cohabitation ; however, the 

 actinian is perhaps a protection to him, since it covers 



