160 ACTINOZOA. 



up of a number of branching tubes, which are not, 

 as in all the preceding forms, perfectly calcareous. 

 In Cornularia and its allies a corallum, never 

 wholly tubular or of a firm calcareous consistence, 

 has yet been detected ; and in Sarcodictyon masses 

 of spicules only can be observed. In some species 

 of Alcyonidce proper, the spicules attain a com- 

 paratively large size, and become aggregated into 

 definite nodular masses. These ' dermosclerites ', 

 as Milne Edwards has shown, are of two principal 

 kinds, the fusiform, and the irregular. The for- 

 mer are somewhat cartilaginous in consistence, 

 and have their surface studded with slight asperi- 

 ties. The irregular nodules are stronger and more 

 decidedly calcareous, presenting six faces, each, in 

 general, furnished with a tubercular enlargement, 

 which sometimes prolongs itself into a number of 

 spines, bearing on their sides other secondary 

 tubercles. By the coalescence of such masses and 

 the deposition of more minute particles among 

 their interstices, a thecal corallum, in other Acti- 

 nozoa, at length comes to be formed. In Alcy- 

 onium itself the spicules, though numerous, are 

 not of large size, and are most conspicuous in the 

 column wall below the margin of the disc. Re- 

 turning to the Zoantharia we find, in the genus 

 Zoanthus, a spicular corallum still more feebly 

 developed than that of Alcyonium. In many of 

 the Sea-anemones no spicules have been observed, 

 though traces of a corallum are not, even in these, 

 absolutely lost. Finally among the Ctenophora 

 we in vain search for the faintest indications of its 

 existence. 



From what has been said it were easy to infer 

 that but little minute structure would be presented 



