ASTEROID AND HELIANTHOID POLYPES. 473 



immersed for a little time in a jar of sea-water, tlie mass swells 

 out again, and from every one of these depressions an eight- 

 armed polype is protruded, "which resembles a flower of ex- 

 quisite beauty and perfect symmetry. In specimens recently 

 taken, each of the petal-like tentacula is seen with a hand-glass 

 to be furnished with a row of delicately slender pinnoe or fila- 

 ments, fringing each margin, and arching outwards ; and with a 

 higher power, these pinnse are seen to be roughened, throughout 

 their whole length, with numerous prickly rings. After a day's 

 captivity, however, the petals shrink up into short, thick, un- 

 shapely masses, rudely notched at their edges" (Gosse). When 

 a mass of this sort is cut into, it is found to be channelled out, 

 somewhat like a sponge, by ramifying canals; the vents of which 

 open into the stomachal cavities of the polypes, which are thus 

 brought into free communication with each other, — a character 

 that especially distinguishes this order. A movement of fluid is 

 kept up within these canals, as may be distinctly seen through 

 their transparent bodies, by means of cilia lining the internal 

 surfaces of the polypes ; but no cilia can be discerned on their 

 external surfaces. The tissue of this spongy polypidoni is 

 strengthened throughout, like that of Sponges (§ 296), with mine- 

 ral spicules (always, however, calcareous), which are remarkable 

 for the elegance of their forms ; these are disposed with great 

 regularity around the bases of the polypes, and even extend part 

 of their length upwards on their bodies. The presence of such 

 spicules is, in fact, a very constant character throughout this 

 group. Thus in the Q-orgonia or Sea-fan, whilst the central part 

 of the polypidom is consolidated into a horny axis, the soft flesh 

 which clothes this axis is so full of tuberculated spicules, espe- 

 cially in its outer layer, that, when this dries up, the spicules 

 form a thick yellowish or reddish incrustation upon the horny 

 stem ; this is, however, so friable, that it may be easily rubbed 

 down between the fingers, and, when examined with the Micro- 

 scope, it is found to consist of spicules of diflerent shapes and 

 sizes, more or less resembling those shown in Eigs. 230-232, 

 sometimes colorless, but sometimes of a beautiful crimson, yel- 

 low, or purple. These spicules are best seen by the methods of 

 illumination that give a black ground, on which they stand out 

 with great brilliancy. They are, of course, to be separated from 

 the animal substance in the same manner as the calcareous spi- 

 cules of Sponges (§ 298); and they should be mounted, like them, 

 in Canada balsam. It is interesting to remark that the hard cal- 

 careous stem of the Red Coral, which takes the place of the horny 

 axis of the Sea-fan, is found, by the examination of very thin 

 sections, to be made up of a solid aggregation of separate spicules, 

 closely resembling those of Alcyonian zoophytes in general. The 

 spicules always possess an organic basis ; as is proven by the 

 fact, that when their lime is dissolved by dilute acid, a gelatinous- 



