330 COELENTERATA ANTHOZOA chap. 



tinguished the free distal portions of the zooids bearing the 

 mouths and tentacles (the "anthocodiae ") from the common colonial 

 mass perforated by the coelenteric cavities of the individual 

 zooids. The coelenteric cavities are separated by a considerable 

 amount of a substance called the " mesogloea," usually gelatinous 

 in consistency but chemically more closely related to mucin than 

 to gelatin, which is traversed by endodermal canals, rods of 

 endoderm cells and a number of free amoeboid cells. In this 

 substance, moreover, there are found in nearly all cases numerous 

 spicules of carbonate of lime formed by the "scleroblasts " (spicule- 

 forming cells) which have wandered from the superficial ectoderm 

 of the common colonial mass. This common colonial mesogloea 

 with its spicules, endoderm cells, and superficial covering of 

 ectoderm is called the " coenenchym." The form assumed by the 

 colonies is very varied. In some species of Clavidaria they 

 form encrusting plates following the irregularity of the rock or 

 stones on which they grow, in Alcyonium they construct lobed 

 masses of irregular form, in Sarcoj)liytwni they are usually shaped 

 like a mushroom, in Jitncella they are long whip-like rods, in 

 most of the Gorgonacea they are branched in all directions like 

 shrubs or in one plane to form fan- shaped growths, and in many 

 of the Pennatulacea they assume that graceful feather form which 

 gives the order its name. 



The consistency and texture of the colonies also varies con- 

 siderably. In some cases where the spicules are few or very 

 small, the substance of the colony is soft to the touch, and 

 frequently slimy at the surface, in other cases the great number 

 of the spicules makes the colony hard but brittle, whilst in a 

 few genera {Sderophytum, Jleliopora) the colony is so hard that 

 it can only be broken by the hand with difficulty. In some 

 genera {Spongodes and the Muriceidae) projecting spicules cause 

 the surface to be rough or thorny, and in the Primnoidae the 

 zooids and the surface of the general coenenchym are protected 

 by a series of overlapping scales or plates. 



In all the Alcyonaria the nematocysts are very minute, and 

 although they can undoubtedly paralyse minute organisms they 

 are unable to penetrate the human skin. None of the Alcyonaria 

 have been described as stinging -corals except the Pennatulid 

 Vinjularia rumphii. 



Zooids. — The fully formed zooids of the Alcyonaria exhibit 



