170 GENERAL PRINCIPLES OF ZOOLOGY. 



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

 The ants keep 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 association, while 

 the other animal is passively led into it. 



An instance of most complete equal rights and true symbiosis is 

 furnished us, however, by a hermit-crab and an actinian (fig. 113), Eupa- 



gurus pubescens and Epizoanthus ameri- 

 canus. Like every species of hermit-crab 

 this also inhabits a snail-shell from the 

 opening of which only his legs and pin- 

 cers are protruded. Upon this shell an 

 Epizoanthus becomes attached and by 

 budding soon covers it with a colony of 



FIG. m-Ao^TTf E^anthus P ^ 8 ' " ^ fter th " s Coring the shell 

 americanus on the shell occu- it is not only capable of extending the 

 $e e rril!) a hermit ' crab - (From aperture by its own growth, but has the 



power of entirely dissolving and absorb- 

 ing the substance of the shell so that no trace of it can be found, though 

 the form is perfectly preserved by the somewhat rigid membrane of the 

 polyp." The advantage which the actinian derives from this symbiosis is 

 clear : it gains a share of the food which the crab obtains. It is less 

 clear what the crab gains by the association ; however, the polyp is perhaps 

 a protection to him, by means of its batteries of nettle cells, while by 

 growth it increases the size of the ' house ' occupied by the hermit and 

 thus saves him periodic changes of abode. 



Occurrence of Symbiosis. That animals in general rarely live sym- 

 biotically with one another rests mainly upon the fact that the conditions 

 of life of all animals to a certain point are similar or identical. They all 

 take in compounds rich in carbon and nitrogen, decompose them, and, in 

 the presence of oxygen, separate them into carbon dioxide, water, and 

 oxidation products containing nitrogen. All animals consequently are 

 competitors in the struggle for food. For the same reason, conversely, 

 symbiosis between plants and animals is not at all uncommon. In 

 particular there are certain lower algaB, the ZooxanthellaB, which often 

 live in animals. The radiolarians contain with such constancy in their 

 soft bodies green- or yellow-colored cells that for a long time these were 

 regarded as constituent parts of the animal. Quite similar yellow and 

 green cells inhabit the stomach epithelium of many actinians, corals, and 

 even of many worms. The ZooxanthellaB are nourished by the carbon 

 dioxide which is formed by the animal tissues, and breathe out oxygen, 

 which in turn serves as food for the animal ; further, they form starch 

 and other carbohydrates, and there is nothing to prevent any surplus thus 



