222 THE BIOLOGY OF THE SEA-SHORE 



general independence of parts from the standpoint of 

 neuromuscular activity. Compared with the behaviour of 

 anemones that of Corymorpha is distinctly less effective. 

 For instance, the tentacles of an anemone when touched by 

 a morsel of food turn in all directions till they have gradually 

 entwined it. At the same time the tentacles secrete a sticky 

 mass and liberate nematocysts with great freedom. Eventu- 

 ally, by the action of cilia and muscles, the food is delivered 

 at the mouth. In Corymorpha, on the other hand, the 

 tentacles respond by waving in only one direction — towards 

 the mouth — and since the response remains unaltered whether 

 food touches their oral or ab-oral surface, the food in the 

 latter case is left behind. The proximal tentacles of Cory- 

 morpha secrete no mucus, and there are no cilia to help 

 waft the food towards the mouth. The behaviour of 

 Tuhularia shows the same lack of correlation and, on the 

 whole, the behaviour of hydroids would thus seem to be on 

 a distinctly less effective level than that of Anthozoan polyps. 



Echinoderms. — The nervous system of this group, as 

 illustrated by the starfish, represents an interesting advance 

 upon that of Coelenterates. What we have referred to in this 

 latter group as the primitive nerve-net is represented in 

 Echinoderms by a plexus of nerve fibres and " ganglion " 

 cells at the base of the ectoderm. This nervous layer, 

 however, becomes distinctly thickened in certain places 

 and causes the ectoderm to project as a ridge. One of these 

 ridges runs down the middle of the under side of each arm 

 and is known as the radial nerve cord. These ridges or 

 radial nerve cords are united by a similar thickening of 

 circular shape on the inner surface of the mouth, known as 

 the nerve-ring. Translating this into the terms employed 

 at the beginning of this section we may say that the nervous 

 organisation of Echinoderms is at that stage where an 

 adjustor element is becoming plainly differentiated. 



The activities of Echinoderms show a high degree of 

 co-ordination, but it is impossible to point to any obvious 

 centre, such as the cerebral ganglion of worms and Arthro- 

 pods, or the brain of higher types. The co-ordinator of the 



