2 32 Elementary Biology. 



we must here briefly describe how the nervous system of the 

 higher animals has probably arisen from the simple elements 

 we find in the Hydrozoa. 



Briefly stated, in the animal groups immediately above 

 the Hydrozoa the tendency is first of all towards elongation 

 of the body and concentration of the sense-organs at the. 

 anterior or head end. Naturally that would lead to aggre- 

 gation of the nervous elements at the anterior end and along 

 either side as being the points of most frequent contact. 

 The head as containing the sense-organs, and therefore being 

 the regulating centre for the entire body, would contain, 

 naturally enough, the largest number of nerve-cells, which 

 would form one or more clumps or ganglia at either side of 

 the mouth, connected by commissures or connecting strands 

 of nervous tissue for the sake of co-ordination ; and from 

 these ganglia nerve impulses would be sent off along the 

 nerve fibres collected in the form of lateral cords or bands 

 along either side of the body. This is the actual arrange- 

 ment in many of the lower worms, and intermediate stages 

 are not wanting connecting such types with the diffused 

 nerve plexus of the Hydrozoa. 



We have thus seen that the hydrozoon begins its life- 

 history as an amoeboid cell ; that by segmentation and 

 foimation first of a one-layered and then of a two-layered 

 sac or planula it passes on to the adult stage by subsequent 

 differentiation. We may now examine a type of which the 

 same is true, but which goes further, and where the embryo 

 is itself segmented. Further, an aperture is opened at the 

 posterior end of the enteronto permit of the more convenient 

 escape of excreta, and special organs are developed out of 

 mesoblast for the performance of duties which each in- 

 dividual cell of Hydra has to perform for itself. These 

 organs are lodged in a space hollowed out of the mesoblast 

 itself, the ccelom, or body-cavity, or in outgrowths from the 

 primitive alimentary canal. 



