CGELENTERATA : ALCYONARIA. 169 



whilst in Renilla the polypes are unilateral, and the polyp- 

 iferous coenosarc is thin and reniform. 



In many of the Pennattilida, as originally shown by Kolliker, 

 the actinosoma consists of two classes of zooids the one com- 

 posed of sexually mature polypes, the other, more numerous, 

 of sexless polypes which have a body-cavity and stomach, 

 but have neither mouth nor tentacles. These sexless zooids 

 may be distributed promiscuously over the whole actinosoma 

 ( Veretillum, &c.), or they may be restricted to definite regions 

 (Pennatula, Virgularid). Whilst many of the Pennatulidcc 

 seem to live habitually sunk partially in the mud of the sea- 

 bottom, others are found freely floating in the water, and their 

 mode of life is not completely understood. 



FAMILY IV. GORGONID^:. In the Gorgonidcz, or "Sea- 

 shrubs," there is an arborescent ccenosarc permanently rooted and 

 provided with a grooved, or silicate, branched sclerobasis, associated 

 with true tissue-secretions, termed " dermosclerites. 



The sclerobasis of the Gorgonidce varies a good deal in its 

 composition. In some it is corneous, and these have often 

 been confounded with the Antipathida, amongst the Zoan- 

 tharia. The distinction, however, between them is easy, when 

 it is remembered that the polypes in the Gorgonida have 

 tentacles in multiples of four, whilst in the Antipathidce they 

 are in sixes. The sclerobasis, too, in the former is always 

 marked by grooves, whereas in the latter it is always either 

 smooth or spinulous. In Isis and Mopsea the sclerobasis con- 

 sists of alternate calcareous and horny segments, branches 

 being developed in the former from the calcareous, and in the 

 latter from the horny segments. 



In Corallium rubrum, the "red coral" of commerce (fig. 

 83), the sclerobasis is unarticulate, or unjointed, and is entirely 

 calcareous. It is the most familiar member of the family, and 

 is largely imported for ornamental purposes. Red coral con- 

 sists of a branched, densely calcareous sclerobasis, which is 

 finely grooved upon its surface, is of a bright-red colour, and is 

 in reality composed of fused spicules. -The corallum is in- 

 vested by a ccenosarc, also of a red colour, which is studded 

 by the apertures for the polypes, which are white, and possess 

 eight pinnately-fringed tentacles. The entire ccenosarc is 

 channelled out by a number of anastomosing canals, which 

 communicate with the somatic cavities of the polypes, and are 

 said to be in direct communication with the external medium 

 by means of numerous perforations in their walls. The entire 

 canal system is filled with a nutrient fluid, containing cor- 

 puscles, and known as the "milk." 



