88o 



SPONGES AND ZOOPHYTES 



but a series of slight depressions arranged with a certain regularity. 

 But after being immersed for a little time in a jar of sea- water the 

 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 fur- 

 nished with a row of delicately slender pinnce or filaments, fringing 

 each margin, and arching onwards ; and with a higher power these 

 pinnae 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, unshapely 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 cha- 

 racter that especially distinguishes this order. A movement of fluid 

 is kept up within these canals (as may be distinctly seen through 



FIG. 667. Spicules of Alcyoniun 

 and Gorgonia. 



FIG. 668. A, spicules of Gorgonia guttata 

 B, spicules of Muricea elongate/. 



their transparent bodies) by means of cilia lining the internal surfaces 

 of the polypes ; but no cilia can be discerned on their external sur- 

 faces. The tissue of this spongy polypidom is strengthened through- 

 out, like that of sponges, with mineral spicules (always, however, cal- 

 careous), 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. In 

 the Gorgonia 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, especially in its outer layer, that, 

 when this dries up, they form a thick yellowish or reddish incrusta- 

 tion upon the horny stem. This crust is, however, so friable that it 

 may be easily rubbed down between the fingers, and when examined 

 with the microscope it is found to consist of spicules of different 

 shapes and sizes, more or less resembling those shown in figs. 667, 668, 

 sometimes colourless, but sometimes of a beautiful crimson, yellow, 



