POLYPI. 127 



detachment of gemmulcs, or imperfectly formed portions of 

 their soft substance. These gcmmules require to undergo 

 the same kind of metamorphosis in order to bring them to 

 their perfect state; and when newly detached from the pa- 

 rent, they exhibit the same singular spontaneous motions, 

 buoying themselves in the water, and swimming in various 

 directions, by the rapid vibrations of their cHla, till they 

 find a place favourable to their growth. On becoming fixed, 

 they spread out to form a base for the future superstructure; 

 and, after the foundation has thus been laid, they proceed in 

 their upward growth, depositing a calcareous or horny axis 

 in successive layers, until it has acquired the requisite thick- 

 ness; and they then gradually assume the forms character- 

 istic of the particular species to which they belong. The 

 materials thus deposited are permanent structures, not ca- 

 pable of modification or removal, and not possessing any 

 vital properties; for these properties belong exclusively to 

 the animated flesh with which these structures are associated. 

 The polypes themselves are not developed till after the for- 

 mation of the root and stem; their growth being in this re- 

 spect analogous to that of the leaves and flowers of a plant. 

 The gemmules of the Flustra carbasea may be selected 

 in illustration of this phenomena. These have been ob- 

 served 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 frequent- 

 ly seen to contract the circular margin of their broad ex- 

 tremity; and, while 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 

 circles, with no other apparent object than to keep them- 

 selves afloat, until they shall arrive at a favourable spot for 



* Edinburgh Phllosoplncal Journal, XYII. 107 and 'oo7. 



