14 



THE CCELOMATE BODY 



With the annelids we meet the type of body structure which is 

 found in all the remaining groups of the animal kingdom. It 

 seems to have been adequate for all the developments required 

 for different types of habit and habitat, and although there are 

 many modifications and some regressions there are no changes 

 in the basic plan. In this chapter the chief characters of the 

 coelomate body, as it is called, are briefly described ; for details 

 the chapters dealing with the various types should be consulted. 

 The basic plan is an arrangement of three layers of tissue, with, 

 in the middle one, a series of spaces. Generally speaking the inner 

 and outer layers are formed in the embryo simultaneously, and 

 before the middle one ; the inner one is called the hypoblast, 

 and the tissues to which it gives rise are known collectively 

 as endoderm. This forms the lining of the gut and little 

 else. The outer layer is similarly called the epiblast, and the 

 tissues which it forms are the ectoderm. This is chiefly the 

 outer parts of the skin (and outgrowths from this such as 

 hair and feathers) but it also includes the nervous system. 

 Between the hypoblast and the epiblast, and generally formed 

 a little later, often from the hypoblast, is the mesoblast. It 

 gives rise to the mesoderm, which includes almost all the other 

 tissues of the body : muscles, gonads, often the excretory organs, 

 and in the vertebrates the skeleton. The three germ layers, 

 as hypoblast, mesoblast, and epiblast are called, are not layers 

 of cells in the sense in which one speaks of layers of chocolates 

 in a box, but in the sense in which the term is applied to a 

 sandwich, to describe plates of material, of different origin, 

 separated from their neighbours ; that is to say, any of the 

 germ layers may be more than one cell thick. In addition to the 

 annelids and those groups which we have not yet described, the 

 flatworms and roundworms have three germ layers ; all these 

 animals are known collectivel}) as Triploblastica, in distinction 

 from the coelenterates, which have no mesoderm and are Diplo- 

 blastica. The gonads of the coelenterates are in the ectoderm or 

 endoderm according to the class, but they have little other 

 specialisation of tissue. The mesogloea is non-cellular, and corre- 

 sponds to the mesoblast in little else besides position. 



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