THE STRUCTURAL BASIS OF THE BODY 



of a common structural basis and can therefore be replaced at a.iv tm 

 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 nectocvste 

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

 kill the smaller living organisms which are 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 ccelenterata 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, t. 

 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 fo. 

 the organism is the sea- water or other medium in which they live, 

 penetrate through the interstices between the cells, and every eel 

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

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

 with the formation of the ccelomata, the class to which all the higl 

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

 internal medium is provided for all the working cells of the fc 



