POLYPI. 169 



until it has acquired the requisite thickness ; 

 and they then gradually assume the forms cha- 

 racteristic of the particular species to which 

 they belong. The materials thus deposited are 

 permanent structures, not capable of modification 

 or removal, and not possessing any vital pro- 

 perties ; for these properties belong exclusively 

 to the animated flesh with which these structures 

 are associated. The polypes themselves are not 

 developed till after the formation of the root and 

 stem ; their growth being in this respect analo- 

 gous to that of the leaves and flowers of a plant. 

 The gemmules of the Fhistra carhasea may 

 be selected in illustration of these phenomena. 

 These have been observed by Dr. Grant,* to swim 

 about in the water as soon as they have escaped 

 from the cells of the parent ; each moving with 

 its narrow end foremost, while the opposite broad 

 end, which is covered with cilia, expands into a 

 flat circular zone. These gemmules are very 

 irritable, and are frequently seen to contract the 

 circular margin of their broad extremity ; and, 

 w hile swimming, to stop suddenly in their course. 

 They swim with a gentle gliding motion, at 

 other times they appear stationary, all the while 

 revolving rapidly round their longer axis, with 

 their broad end uppermost : they often bound 

 forwards, either in straight lines, or describing 



* Edinburgh Philosophical .lournal, XVII. 107 kiuI 337. 



