158 



THE SEA SHORE 



We have not to look far into the structure of any typical 

 echinoderm to see that it is a distinct advance on the anemones in 

 the matter of organisation. To begin with its digestive system 

 this consists of a tube having no communication with the general 

 body-cavity, but remaining quite distinct throughout its length, 

 with both ends communicating directly with the exterior. Its 

 nervous system also is more highly developed, for it has a well- 

 formed ring of nerve matter round the mouth, from which pass two 

 or three systems of nerve fibres, each system having its own special 

 function to perform. The sense organs, however, do not appear to 

 be well developed, though there exist certain ' pigment spots,' in 



which nerve fibres terminate, 

 and which are supposed to serve 

 the purpose of eyes. 



One of the most interesting 

 features in connection with the 

 echinoderms is undoubtedly the 

 structure and function of the 

 apparatus for locomotion. Ex- 

 amine a live sea urchin, or the 

 common five-rayed starfish, in a 

 rock pool or aquarium, and it 

 will be seen to possess a large 

 number of soft, flexible, and 

 protrusible processes, each of 

 which terminates in a little 

 sucking-disc that enables the 

 animal to obtain a good 'foot- 

 hold ; ' and, having fixed itself on one side by means of a number 

 of these little 'feet,' it is enabled, by the contraction of certain 

 muscles, to pull itself along. 



The little feet we are examining are really tubes filled with 

 water, and capable of being inflated by the injection of water into 

 them from within the body of the animal. Each one communicates 

 with a water tube, several of which (usually five) radiate from a 

 circular canal of water that surrounds the mouth. This circular 

 canal does not communicate with the mouth, but with a tube, 

 known as the ' stone canal ' because of the carbonate of lime 

 deposited within its walls, that opens at the surface of the body on 

 the opposite side, and is guarded at the orifice by one or more 

 perforated plates through which water gains admission. Thus the 



FIG. 104. LAKVA or THE 

 BRITTLE STARFISH 



