CHAPTER IV 



ANNELIDA 



In the simple metazoan type represented by Hydra we saw that the 

 body consists of the two primary cell-layers or epithelia — the ectoderm 

 and the endoderm. 



In the slightly more complex type represented by the sea-anemone 

 the body still consists for the most part of the two primary layers 

 but there are now apparent two advances in detail, (i) The endo- 

 derm bulges outwards to form the lining of the recesses or pockets lying 

 between the mesenteries and (2) the mesogloea, the jelly-like material lying 

 between the two primary layers, has become colonized by cells which have 

 wandered into it by amoeboid movement. These immigrant amoebo- 

 cytes constitute a new element in the structure of the body known 

 as the mesenchyme. 



In the still more complex type of structure characteristic of the great 

 majority of the more highly evolved animals, grouped together under 

 the common name Coelomata, we find each of the two peculiarities just 

 mentioned showing further development. The endoderm pockets have 

 become separated off to form a body - cavity, known as the eoelome, 

 surrounding a central tubular enteron or alimentary canal. The 

 lining of these coelomic cavities, originally part of the endoderm, 

 is now distinguished from the definitive endoderm (lining the enteron) 

 under the name mesoderm : it gives rise to the greater part of the 

 muscles of the body and of the excretory and reproductive organs. 



The mesenchyme again has become much more abundant. Indeed 

 in the more bulky animals mesoderm and mesenchyme are responsible 

 for by far the greater part of the body. Apart from the nervous 

 system, which may be of considerable bulk, the ectoderm is confined 

 to a thin layer covering the surface of the skin while the endoderm is 

 similarly confined to a thin layer lining the alimentary canal. 



Not only has the mesenchyme increased greatly in quantity : it 



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