PKOTOZOA. 83 



a more irregular manner, the skeleton is sponge-like. The infinite 

 variety of form is increased by discoidal and basket-like skeletons ; 

 or these skeletons may have a spiral arrangement. In this way a 

 supporting organ of great complexity is formed, in which the soft 

 parts of the body are embedded, and the separate pieces of which 

 are developed in the protoplasm. 



§ 66. 



Compared with these internal organs of support in the Rhizopoda 

 the tests of the Infusoria form a distinct series of arrangements, 

 for they are only secretions from the surface of the body : they 

 resemble the tests of the Arcellas, mentioned above. The secreting 

 matrix is in this case an anatomically distinct part of the body. But 

 this need not be regarded as a higher stage, for it is closely allied to 

 the lowest, i.e. to the formation of a cell-membrane. Tests are 

 principally developed in the fixed Infusoria. They are formed by 

 the secretion of a substance, which is primitively soft, and which 

 gradually hardens ; this surrounds the body of the animal like a cup 

 or an urn, except at one point, where it allows of communication 

 with the exterior. These tests are distinguished from the merely 

 cuticular structure, which tends to form a carapace, in that the 

 differentiated layers attain greater firmness, and become separated 

 from the greater part of their matrix surface. But the genesis of 

 both structures is the same. It is identical with the formation 

 of cysts, a process which is very common among the Infusoria, and 

 by means of which the organism shuts itself off from the exterior 

 for a time, so as to withstand unfavourable conditions (loss of water, 

 and so on). The immovable stalk of Epistylis, and the external 

 layer of the contractile stalk of the Vorticellinao and Carchesinaa 

 must be regarded as cuticular differentiations of this kind. The 

 tests maybe soft or hard, and membranous. Some are distinguished 

 by the agglutination of foreign bodies — cemented grains of sand, 

 and so on. The genera Vaginicola, Tintinnus, etc., have shells. 

 Stentor has one in certain cases. Fenestrated shells have also 

 been observed (Dictyocyrta). A carapace is formed from the firm 

 hyaline cuticle in Stylonychia, Euplotes, Aspidisca, Spirochona, 

 Coleps, etc. 



§ 67. 



Organs for the prehension and alteration of food are 

 wanting in the lowest organisms. In the Gregarinas food is taken 

 in by endosmotic processes at the surface, and solid nutritivo 

 matters do not reach the interior of the body. Where the body 

 is not peripherally differentiated there is, on the other hand, a 

 direct taking in of food, which may go on at any part of the 

 body. This is the case in the Rhizopoda. In this case the nutritive 

 matter is surrounded by the soft substance of the body, or it is 

 embraced by the processes of the body — the pseudopodia. In both 



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