THE STRUCTURAL BASIS OF THE BODY 33 



of a common structural basis and can therefore be replaced at any time by 

 the cytoplasm if destroyed. Examples of such organs are the cilia, the 

 commonest motor apparatus of unicellular organisms ; the pseudopodia, 

 which, as we have seen, can be made and destroyed at will ; the mouth of 

 animals such as Volvox or Vorticella ; and the stinging cells or nectocysts, 

 which surround the mouth of many of these animals and serve to paralyse 

 or kill the smaller living organisms brought by the cilia within reach in 

 order that they may serve as food. In contradistinction to these organs 

 are (2) a number of others which must be regarded as permanent. These 

 cannot be formed by differentiation from the cytoplasm of the cell, but are 

 derived by the division of pre-existing organs of the same character, and 

 are therefore transmitted from one generation to another. As examples of 

 such cell-organs may perhaps be mentioned the nucleus, with its chromo- 

 somes, and the plastids, of which the chloroplasts of vegetable cells are the 

 most conspicuous. Certain cell organs may fall into either class. Thus 

 the contractile vacuoles are sometimes derived by the division of the pre- 

 existing vacuoles in a previous generation, at other times are certainly formed 

 out of the common cytoplasm. The centrosome, a small particle generally 

 situated in the cytoplasm, which plays an important part in cell-division, 

 is generally derived by the division of a pre-existing centrosome, but under 

 certain conditions and in some organisms can be developed in situ in the 

 cytoplasm itself. 



The possibility of histological differentiation and of the adaptation of 

 structure to definite functions becomes much more pronounced as we pass 

 from the unicellular to the multicellular organisms or metazoa. The lowest 

 of the metazoa, such as the sponges, consist of little more than an aggregation 

 or colony of cells. All the cells are still bathed with the outer fluid, and any 

 differentiation of structure or function seems to be entirely conditioned by 

 the position of the cell. In the coelenterata the differentiation is already 

 much more marked. The hydra, one of the simplest of the group, consists 

 of a sac formed of two layers of cells and attached by a stalk to some firm 

 basis. Round the mouth of the sac is a circle of tentacles. The inner layer, 

 or hypoblast, represents the digestive and assimilatory layer, while the 

 epiblast, or outer layer, is modified for the purposes of protection, of reception 

 of stimuli, and of motor reaction. In the jelly-fish the differentiation of the 

 outer layers leads to the formation of the first trace of a nervous system, i.e. 

 a system fitted especially for the reception of stimuli and for their trans- 

 mission to the reactive tissues, namely, the muscles. 



In all these classes of animals the external medium of every cell forming 

 the organism is the sea- water or other medium in which they live. This can 

 penetrate through the interstices between the cells, and every cell is there- 

 fore exposed to all the possible variations which may occur in the composition 

 of the surrounding medium. A great step in evolution was accomplished 

 with the formation of the coelomata, the class to which all the higher animals 

 belong. In these, by the formation of a body cavity containing fluid, an 



Kternal medium is provided for all the working cells of the body. The com- 

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