HALICHONDRIA. 129 



radiating from the centre to the margin of the papillae, and at 

 length a minute circular opening is perceived in the centre of 

 the villous surface. The papilla advances from the shell, and 

 its central opening enlarges in proportion to the healthy state 

 of the zoophyte, and the purity and stillness of the water ; its 

 flat downy radiated surface gradually diminishes by the widen- 

 ing of the central opening, till only thin margins are left around 

 the orifice, and the current is again seen to play briskly from it. 

 In recent specimens of the Cliona dredged from an oyster-bed 

 near the shore at Prestonpans, I examined under the most fa- 

 vourable circumstances on the coast, I have twice observed po- 

 lypi of extraordinary minuteness and delicacy placed around the 

 margin of the orifice, and which, kept in constant motion, ad- 

 vancing and withdrawing themselves into the substance of the 

 papilla, while the current flowed from its central opening. The 

 polypi were perfectly invisible to the naked eye in an ordinary 

 light and position ; but by suspending the Cliona in a crystal 

 jar with clean water, and placing it between the eye and a can- 

 dle, or the sun, they were seen like filaments of silk or asbestus 

 constantly rising and sinking on the margin of the papilla. On 

 cutting off a papilla, and placing it under a microscope in sea 

 water, the polypi continued their motions, and were seen to con- 

 sist of a long, slender, transparent, cylindrical, tubular fleshy 

 body, at the farther extremity of which were placed about eight 

 short broad tentacula, slightly dilated at their free ends, which 

 were constantly inflecting and extending themselves irregularly 

 while the polypi advanced or retreated. In two entire and fresh 

 specimens, the polypi continued visible and in motion for more 

 than twenty -four hom's in a jar of water at Prestonpans ; but I 

 have not yet succeeded in perceiving them in any of the nume- 

 rous specimens which I have preserved alive in the water pro- 

 cured from Newhaven." 



Notwithstanding this circumstantial history, the accuracy of 



I 



