SECT. iv. THREAD-CELLS OF ACTINIA. 133 



-water which fills the perpendicular chambers is forced in 

 a stream through the slits, and carries with it the white 

 filaments lodged within them ; and then these quivers, 

 which are full of deadly weapons, are ready for action. 



Under the microscope, the white filaments are like 

 narrow flat ribbons with their edges curled in, and 

 thickly covered with cilia. They have not the slightest 

 trace of muscular fibre, even .when viewed with a mi- 

 croscopic power of 800 diameters ; yet they extend, 

 contract, bend, and coil in every direction; they bring 

 together the margins of the ribbon so as to form a 

 tube, and open them again ; and the filaments perform 

 all these motions even when severed from the animal, 

 no doubt by the contractile nature of the clear jelly 

 or sarcode, of which their bases are composed, as in the 

 tentacles of the Acalephae. 



Innumerable oblong dart or stinging-nettle cells, 

 closely packed together, lie under the folded edges of 

 the ribbons, throughout their whole length, especially 

 at their tips. 1 



The polypes of the stony corals, though extremely 

 small, are essentially the same in structure as the Sea 

 Anemone, but they have no sucker at their base. The 

 Sea Anemone is of soft tissue throughout its whole body. 

 In the polypes of the madrepore corals, on the contrary, 

 the whole of the perpendicular lamellae which divide 

 the interior of the body into chambers become hard, 

 from being consolidated by particles of carbonate of 

 lime; and their upper edges, which appear as rays round 

 the mouth of the animal, give that stariy appearance 

 to the surface of dead madrepores after the soft part of 

 the polypes has been destroyed. 



Most of the coral polypes are unarmed; but in some, as, 

 for example, the Caryophyllia Smithii, there are multi- 

 tudes of dart-cells in the tentacles, besides numerous 



1 ' Evenings at the Microscope,' by P. H. Gosse, Esq. 



