ECOLOGY 279 



dependent condition and many of them have undergone such extensive 

 changes in their organization as to make them incapable of hfe elsewhere 

 than on or in the host. The following statement of the degeneration 

 associated with internal parasitism, taken from Hertwig's Manual of 

 Zoology, p. 168, will make this clear. 



"The degree to which a parasite has become dependent upon its host varies 

 in the different species; it is determined by the extent to which the parasite has 

 adapted itself to the organization of its host. Therefore it is necessarj^ when 

 speaking of parasitism to consider the changes of form which the parasitic mode 

 of life has caused in the structure of animals. These concern most immediately 

 the organs of locomotion and nutrition. Since a parasite needs to fix itself as 

 firmly as possible to the host, the locomotor apparatus more or less completely 

 disappears and an apparatus for fixation to the host becomes necessary ; parasites 

 of different groups are provided with hooks, claspers, sucking disks, etc. The 

 blood, tissue-fluids, or liquid food of the host furnishes nourishment to the para- 

 site; these are substances in solution which scarcely need digestion. Usually, 

 therefore, the digestive canal is simplified or quite disappears; among the para- 

 sites there are gutless worms as well as gutless Crustacea. The mode of life of a 

 parasite is also simplified, since it is no longer compelled to seek its food; in all 

 parasites the nervous system and sense organs undergo a high degree of degenera- 

 tion; the former becomes limited usually to the most indispensable portion; the 

 latter, except those of touch, may entirely disappear." 



There are two classes of parasites, those living within the host, 

 entoparasites, and those living on the outside of the body, ectoparasites. 

 Examples of entoparasites are tapeworms, which live in the alimentary 

 canal, and liver-flukes, which occur in the liver; and among the ecto- 

 parasites may be mentioned lice, fleas and mites. 



Symbiosis. — If the individuals of different species stand in close 

 reciprocal relation the association is known as symbiosis and the associated 

 animals are called symhiotes. Symbiosis is rare among animals, a classic 

 example being that of the hermit crab, Eupagurus pubescens, and an 

 actinian, Epizoanthus americanus. 



"Like every species of hermit crab this also inhabits a snail shell, from the 

 opening of which only his fore legs and pincers are protruded. Upon this shell an 

 Epizoanthus becomes attached and by budding soon covers it with a colony of 

 polyps. 'After thus covering the shell it is not only capable of extending the 

 aperture by its own growth, but also has the power of entirely dissolving and 

 absorbing 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 from periodic changes of abode." 

 (Hertwig, Manual of Zoology, p. 170.) 



