244 HYDROID POLYPES LESS. 



the same thing, as a Hydra, the umbrella has undergone a very 

 remarkable differentiation of its tissues. Its ordinary ecto- 

 derm cells, instead of being large and eminently contractile, 

 form little more than a thin cellular skin or epithelium over 

 the gelatinous mesogloea : they have largely given up the 

 function of contractility to the muscle processes or fibres, 

 and have taken on the functions of a protective and sensitive 

 layer. 



Similarly the function of automatism, possessed by the 

 whole body of Hydra, is made over to the group of specially 

 modified ectodermal cells which constitute the central 

 nervous system. If a Hydra is cut into any number of 

 pieces, each of them is able to perform the ordinary move- 

 ments of expansion and contraction, but if the nerve-ring 

 of a medusa is removed by cutting away the edge of the 

 umbrella, the rhythmical swimming movements stop dead : 

 the bell is in fact permanently paralysed. 



It is not, however, rendered incapable of movement, for 

 a sharp pinch, i.e. an external stimulus, causes a single con- 

 traction, showing that the muscles still retain their irritability. 

 But no movement takes place without such external stimulus, 

 each stimulus giving rise infallibly to one single contraction : 

 the power possessed by the entire animal of independently 

 originating movement, i.e. of supplying its own stimuli, is 

 lost with the central nervous system. 



Another instance of morphological and physiological 

 differentiation is furnished by the pigment spots or ocelli 

 (Fig- 53) c > oc) situated at the bases of the tentacles. They 

 consist of groups of ectoderm cells in which are deposited 

 granules of deep red pigment. Their function is proved by 

 the following experiment. 



If a number of medusae are placed in a glass vessel of 

 water in a dark room, and a beam of light from a lantern is 



