Evolution 



special characteristic is that there is a great mass of gelatinous 

 substance between the outer and inner cell layers. The repro- 

 ductive cells, as in the sea-anemones, are produced by the inner 

 cell layer, and escape by the mouth. 



Something remains to be said regarding the specialisation of 

 tissues in this group. We have already mentioned the stinging 



Fig. 36. — Diagrammatic section of Medusa. 



cells, and the beginnings of muscular tissue, in Hydra. The 

 former are a constant feature of the Coelenterates, while the latter 

 reaches a very considerable development in the higher forms, as 

 may be judged from the surprising rapidity with which the Medusa 

 can swim, or from the strength with which the sea-anemone 

 can retract its tentacles and draw itself together. Important, 

 further, is the nerve tissue. This consists of cells whose business 



50 



