Dr Grant on the Structure and Nature ofFlustra. 113 



part of a line in length, of equal thickness throughout, round at their free 

 extremities, and dissolve with effervescence in diluted nitric acid. They are 

 not perceptible without the aid of a microscope. The spicula are arranged 

 along the whole of each side of the cell ; they are placed in nearly parallel 

 groups, of three or four, at short distances from each other, and are most 

 numerous at the middle of the cell where the principal part of the polypus 

 is usually coiled up in a spiral turn. 



In the newly formed cells at the extremities of the branches, we at first ob- 

 serve the spicula only at the part of the cell where the body of the young po- 

 lypus is still entirely shut up in a sac. The cell is usually shut, or nearly 

 shut, at the top, in the retracted state of the polypus, but opens by a kind of 

 semilunar valve, with firm margins, when the polypus is advancing out from 

 the aperture. The back of the cell is formed by a transparent tough membrane, 

 which contains some opaque spots of calcareous matter, and exhibits numerous 

 transparent branched lines, like vessels or fibres, running chiefly in a longi- 

 tudinal direction. When the polypus is dead, and nearly absorbed, many of 

 these vessels are seen radiating from the last remains of the polypus, which 

 appear as a small red or brown spot in the centre of the posterior wall of the 

 cell. The lateral walls of the cell appear to consist of a thin calcareous lami- 

 na, lying perpendicularly to the general plane of the cells, it is white, and very 

 tough ; and, when highly magnified, it exhibits fibres or vessels, running lon- 

 gitudinally on its surface. Mr Ellis supposed the lateral walls of the cells of 

 Flmtrce to be formed by a tube. When we look perpendicularly on this 

 part, it appears as a white filament ; but when viewed laterally, we observe it 

 to consist of a regular thin plate, surrounding the whole margin of the celL 

 By examining carefully with the microscope the margins and corners of the 

 cells, we observe, that there is a thin transparent membranous lining within 

 the walls of the cell. In the young cells, this internal lining forms a small 

 shut sac at the bottom of the cell, in which the infant polypus is inclosed and 

 matured : this sac gradually extends to the aperture of the cell through which 

 the polypus at length protrudes its tentacula ; and, at last, it is found nearly 

 applied to the walls of the cell. The particles of sand and other matter, which 

 sometimes appear to be within the cells, are generally on the outside, adher- 

 ing to the posterior wall. 



The polypus of the F. carhasea is nearly twice as long as the cell which con- 

 tains it, and when retracted within the cell, it is found coiled up in a spiral 

 turn, extending from the aperture to the base of the cell. The polypus con- 

 sists of the tentacula, the head, the body, and a large globular appendix, at- 

 tached to the posterior part of the body. The tentacula are usually twenty- 

 two in number, sometimes we observe only twenty-one ; they are long, slen- 

 der, cylindrical, of equal thickness throughout, and have each a single row of 

 cilice^ extending along both the lateral margins from their base to their free 

 extremity- The tentacula are nearly a third of the length of the body of the 

 polypus, and there appear to be about 50 cilise on each side of a tentaculum, 

 making 2200 cilise on each polypus. In this species there are more than 18 

 cells in a square line, or 1800 in a square inch of surface, and the branches of 

 an ordinary specimen present about 10 square inches of surface ; so that a com- 

 mon specimen of the F. carhasea presents more than 18,000 polypi, 390,000 

 APRIL — JUNE 1827. * H 



