vii ORIGIN OF PRIMITIVE ORGANS 325 



send forth pseudopodia to take up food ; but its flagella are 

 retractile, as all flagella, being derived from pseudopodia, 

 must have been originally, and as in many other cases they 

 are still, e.g. in the Flagellata. 



Again, it must have been in consequence of function (use) 

 that the endoderm cells gradually came to differ from those of 

 the ectoderm in size and internal structure, and when the 

 gastrula developed into a fixed sessile animal, disuse caused 

 the disappearance of the flagella. 



As there are now living forms which retain the morula 

 and blastula form, so there are others which exhibit the 

 gastrula form, although the gastrula has undergone various 

 modifications. Hydra is such an animal. 



In Hydra it is also evident how a third primitive organ of 

 animals arose, namely, the mesoderm. 



In Hydra and many allied forms the ectoderm cells con- 

 sist of two parts, an external, which receives stimuli, and an 

 internal, which is muscular (neuro- muscular cells). This 

 differentiation also cannot possibly have arisen except as a 

 result of function, of activity. In other animals the separate 

 layer of supporting and muscular cells forming the mesoderm 

 arose by growth and multiplication towards the interior, by 

 the separation of special cells from the ectoderm and endoderm. 



It must be described as a special peculiarity belonging to 

 the organisation of a portion of the mesoderm cells, that there 

 " crystallises " in them calcareous or siliceous or horny matter, 

 the first especially in perfectly definite, and often extremely 

 complex, shapes. 



The formation of the supporting skeleton of these lower 

 animals, therefore, has probably not been determined by use 

 or function, but rather by definite laws of development, and 

 selection; but this applies, not so much to the origin of 

 definitely shaped calcareous or siliceous spicula, as to the 

 solidity of the skeleton in general. 



