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GENERAL PRINCIPLES OF ZOOLOGY 



that representatives of the most diverse groups take on a remarkable 

 similarity of appearance and structure. Pcntastomum (fig. -113), 

 for example, belongs in the same class with the spiders, the Arachnida, 

 but in external appearance it is entirely unlike them, resembling 

 the tape-worms (fig. 112). Hence for a long time all entoparasites, on 

 account of their similarity, were united into a single systematic group 

 under the name of 'Helminthes,' comprising members of the Crustacea, 

 worms, and spiders. Only by embryology was the unnaturalness of 

 this grouping recognized. Entoparasitism therefore is one of the best 

 examples for illustrating convergent development, i.e., animals of different 

 systematic position acquiring, under similar conditions of life, a great 

 similarity of structure and appearance. 



Symbiosis. Less frequent than parasitism is symbiosis, or the association 

 of animals for reciprocal advantages. Social animals frequently not only hold 

 certain animals in bondage, but even seek to protect and serve them; as, for 

 example, certain blind beetles, like Claviger or some species of plant-lice, 

 (myrmecophiles) or even ants of other species and genera. But such cases of 

 association correspond in part to the domestication of animals, or to slavery, 

 as carried on by man. The ants keep the plant-lice in order to lick the sweet 

 juice ('honey dew') which is secreted in their honey-tubes; they steal the pupas 

 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 is passively led 

 into it. Symphyly is close to true symbiosis. Besides Claviger, mentioned above, 



many other insects, mostly beetles, which are 

 cared for by the ants, are found in colonies of 

 ants and termites, since they have a sweet 

 secretion on special bundles of hairs, which 

 the ants lick off. Frequently the beetles eat 

 the younger stages of the ants. 



An instance of most complete equal rights 

 and true symbiosis is furnished us, however, 

 by a hermit-crab and an actinian (fig. 114), 

 Eupagiirus pubescens and Epizoantlius attieri- 

 (-(iiiiis. Like all hermit-crabs this also inhabits 

 a snail-shell from the opening of which only 

 his legs and pincers are protruded. Upon 

 this shell an Epizoantlius becomes attached and by budding soon covers it with 

 a colony of polyps. 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; however, the polyp is perhaps a protection to him by 

 its 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 rarely live symbiotically with 

 one another rests largely because the conditions of life of all animals to a certain 

 point are similar or identical. They take in compounds rich in carbon and 

 nitrogen, decompose 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 so uncommon. There are certain lower algae, the Zooxanthellae, 



FIG. 114. A colony of Epizo- 

 anthus americanus on the shell 

 occupied by a hermit-crab (from 

 Yerrill). 



