224 THE BIOLOGY OF THE SEA-SHORE 



has been confirmed by Cowles (191 1). In all three groups, 

 starfishes, sea-urchins, and brittle-stars, the light sense would 

 appear to have its seat in the whole skin. 



The activities of spines and pedicellariae in the sea-urchin 

 have already been alluded to (see pp. 88 et seq). Though 

 their behaviour is of a purely reflex character (a spine or 

 pedicellaria removed from the test, along with a small portion 

 of the underlying skin, will continue to respond to stimulation), 

 yet these structures act together wonderfully harmoniously, 

 so that the sea-urchin has been called a " republic of 

 reflexes." 



On the whole, it would appear that the behaviour of 

 Echinoderms, notwithstanding its predominantly reflex 

 character, is yet exceedingly varied and flexible and is, 

 moreover, capable of being modified by experience. Thus 

 starfishes, when pegged down several times in succession 

 in the same position, showed a gradual reduction of the 

 movements necessary for escape. Again, a short piece of 

 rubber tubing, when passed several times over the arm of 

 a brittle-star, was observed by Preyer to be removed in five 

 different ways. 



Bohn (1908), however, who has repeated Preyer's 

 experiments, has been unable to obtain any confirmation of 

 this whatsoever. The escape of the arm from the rubber 

 tubing is said to be purely a matter of chance. 



Annelids and Crustacea. — The plan of the nervous system 

 is approximately the same in both these groups. Typically, 

 it consists of a pair of ganglia in the head region (cerebral 

 ganglia) united by a ring of nerve tissue to a similar pair of 

 ganglia placed beneath the anterior end of the gullet (suboeso- 

 phageal ganglia). The lower ganglia form the most anterior 

 pair of a double chain of ganglia extending the whole length 

 of the body and united by strands of nerve tissue. In the 

 earthworm or in Nereis^ which we may take as typical of 

 AnneUds, there is a pair of ganglia to each segment, and from 

 each ganglion nerve fibres extend to the skin. In Crustacea 

 such as the crab, successive ganglia tend to fuse, which means 

 that the activities of these forms are more highly centralised 



