Dr Grant on the Structure and Nature ofFlustra. Ill 



species of Flustrae most abundant in the Frith of Forth, and from 

 which the following observations have been chiefly taken, are 

 tlie F.Jbllacea, a branched species with a double plane of cells 

 and two projecting spines at each side of the apertures of the 

 cell ; F. truncata a very delicate branched species, with a double 

 plane of cells disposed in longitudinal straight lines, the sides of 

 the cells are nearly straight and parallel, and have no project- 

 ing spines ; F. carbasea^ a delicate, broad leaved, branched spe- 

 cies, with a single plane of large transparent cells, without pro- 

 jecting spines ; F. dentata, a sessile species, with a single plane 

 of cells, generally incrusting the leaves of large fuci, the mar- 

 gins of the cells are surrounded with numerous short projecting 

 sharp calcareous spines ; the F. pilosa, a delicate sessile species, 

 the apertures of whose cells are defended only by a single long- 

 curved spine, it generally encrusts the stems of the smaller fuci 

 or thQ branches of zoophytes ; and the F. telacea, a sessile spe- 

 cies, with long quadrangular cells covering the leaves of large 

 fuci, the cells having two short spines at their summit. 



When we look through a branch of the F. foliacea or other species of Flus- 

 tra, which has a double plane of cells, we find that the boundaries of the cells 

 on one side, do not coincide with the boundaries of those on the opposite 

 plane ; the position of the cells on one side of the Flustra has no relation to 

 those on the opposite side, and the appearances, presented by the contiguous 

 cells on the opposite sides, are often totally different, the cell on one plane 

 presenting a polypus in full activity, while the contiguous cell on the oppo- 

 site plane presents an ovum arrived at maturity, with the remains of a decay- 

 ed polypus nearly absorbed. This not only produces a confused appearance 

 in the cells, but likewise diminishes their transparency ; and although, in such 

 species, we can tear the two planes of cells separate from each other, this is 

 generally attended with injury both to the cells and their contents. Such 

 species, therefore, though the largest, the most abundant on our coasts, and 

 those which have been most frequently examined, are ill adapted for the 

 commencement of an inquiry of this kind, and the sessile species, which spread 

 as a crust on the surface of opaque bodies, are still more unsuitable. The 

 F. carbasea of Ellis, Lamouroux, Lamarck, &c. is a branched species, which 

 not only has the advantage of being very common on our coasts, and of hav- 

 ing tile cells arranged in a single plane, but likewise of having the cells of a 

 large size, and very transparent, from the small quantity of lime in their 

 parietes. This species is not found near the shore like the F. dentata, F. pi. 

 losa^ and F. telacea, but is brought up in great quantities during the dredging 

 season, from almost all the oyster-beds of the Frith of Forth, where it is 

 found in ramified tufts, from two to four inches high, adhering by a very 

 narrow base to the surface of shells, stones, fuci, and even of the smallest 



