BEHAVIOUR OF SHORE ANIMALS 219 



usually, however, in Ccelenterates a third type of cell is 

 intercalated among the branches of the nerve-net, adding 

 to its complexity and bringing the nervous organisation a 

 step nearer to the centralised system of higher animals. 

 These cells have been called ganglion cells, a term which 

 Parker thinks is inappropriate. 



A nervous organisation such as that just described is 

 characterised by its general diffuseness and lack of centralisa- 

 tion ; points which bring it into sharp contrast with the highest 

 type of nervous system, where a central nervous organ or 

 adjustor is differentiated. According to Parker, this new 

 adjustor element, which plays so important a part in corre- 

 lating and controlling behaviour, has arisen from the nerve- 

 net which, as we have seen, forms an indirect connection 

 between receptors and effectors. The nerve fibres consti- 

 tuting the plexus, originally situated in the outer layers of 

 the animal, tend to become grouped into distinct tracts or 

 bands of tissue, which gradually become separated from the 

 ectoderm and sink into the deeper parts of the animal, 

 giving rise to the ventral nerve-cord characteristic of the 

 higher invertebrates, such as worms and crustaceans. 



This brief outline of the evolution of the primitive 

 nervous system, for which we are indebted to Parker, will 

 serve as a basis for a more detailed discussion of the nervous 

 organisation of the most important shore groups. 



Sponges. — These, the most inert of all the multicellular 

 animals, are devoid of any observable nervous elements 

 whatever. They display, however, a diffuse contractility 

 due to the presence of contractile cells or myocytes. The 

 principal activity of sponges is that of maintaining a con- 

 tinual flow of water through the body by means of the 

 action of the flagellate cells which occur in their interior. 

 The only behaviour observable is when, at low tide or in 

 other unfavourable circumstances, the sponge {e.g. Stylo- 

 tella, or the British genus Pachymatismd) closes its oscula 

 and contracts its flesh. When conditions are again favourable 

 opening occurs as the result of the elasticity of the sur- 

 rounding flesh. The muscle elements act as unco-ordinated 



