Chap, in.] HIGHER METAZOA. 53 



gradually become elaborated. The body is now too 

 large be to able to do without an apparatus by means of 

 which the nutrient material obtained by digestion, or 

 the store of oxygen necessary for the activity of the 

 protoplasm of its constituent cells, may be carried 

 about from part to part, and we have therefore a 

 system of circulating vessels. In many, also, the 

 firm covering of the body necessitates the develop- 

 ment of special outgrowths into which the vessels 

 pass, charged with the carbonic acid which is con- 

 stantly associated with the activity of living proto- 

 plasm ; in these outgrowths the blood gives up 

 carbonic acid, and receives oxygen in its place ; in 

 other words, a respiratory is added on to a 

 circulatory apparatus. In the majority, again, the 

 body is too large to be able to move about without 

 the assistance of special muscular processes or limbs, 

 and these are not unfrequently strengthened and sup- 

 ported by those chitinous secretions which we call 

 setae (bristles). 



Elaborate and complex activities of such a kind 

 as these require to be brought into relation with one 

 another, or, in other words, to be co-ordinated, 

 and performed in regular and systematic fashion ; it 

 is not now sufficient for the organism that there 

 should be a pnestomial nervous mass with some few 

 nerve-fibres eiven off from it. Centres of nervous 



O 



activity must be developed in various parts of the body, 

 and we find, therefore, that collections of nerve-cells 

 are found in different metameres : these ganglicilic 

 masses are connected together by fibres, and so 

 it results that there runs down the ventral surface 

 of the body a chain of ganglia. From each 

 of these ganglia nerve-fibres pass to the muscles 

 and other organs of the body, and to them there 

 come other fibres which have one end in the 

 skin, and which convey to the central apparatus 



