108 THE EVOLUTION OF CONTINUITY 



Each radial canal gives off on both sides a series of short 

 lateral branches, which are continued into the protrusible 

 tube-feet by whose means the animal moves slowly about ; 

 protrusion being the result of increased intra-canal pressure, 

 assisted by special muscles and valves. 



The nervous system takes the form of a circular nerve- 

 tract immediately below the water-vascular ring just men- 

 tioned. From it radiate five nerve-cords, one down each 

 arm below the radial water-vessel. A fine layer of nerve 

 cells also underlies the body ectoderm generally. The 

 reproductive organs are multiple ; in the male ten testes, 



Fig. 38. — Comparing the Starfish with the Medusa, a, 

 vertical section in the line of the arm of a Starfish, in, mouth ; 

 «, stomach ; p, pyloric sac, giving off pc, pyloric caeca ; i, intes- 

 tine ; a, anus ; j, jelly connective tissue ; en, endoderm ; co, 

 coelom ; re, radial canal ; cc, circular canal ; no, nerve-cord. 

 The tube-feet arising from re are not indicated. B. vertical 

 section of a Medusa, dividing a radial canal, m, mouth ; 

 s, stomach ; re, radial canal, a continuation of **, and linked 

 up with cc, circular canal ; nc, nerve-cord ; cc, ectoderm ; /, 

 jelly-tissue ; en, endoderm. 



in the female ten ovaries, situated in pairs at the bases of 

 the arms. 



It is difficult to classify the Echinoderm in terms of 

 living Individual Continuity. The type we have taken, 

 the Starfish, is clearly a higher animal form than the 

 monomegazooidal Medusa. It presents, however, certain 

 characters which seem to indicate some primitive medusoid 

 ancestry. 



For its body tissues are ectoderm, jelly connective tissue, 

 and endoderm, and it has a radial water-canal system linked 

 to a circular canal surrounding the mouth. Further, there 



