INTRODUCTION. 29 



with the assistance of the currents in the water ; when it comes in contact with a hard 

 substance, or rests, out of a current, on soft ground, the base adheres, and the minute 

 tentacular disc is gradually developed^ and finally expands. The young polypes are 

 carried here and there ; they exercise no volition, and only those which find a fit base upon 

 which to rest live on to maturity. Either the young corallum adheres fixedly through life, 

 or is so buried in mud or sand as to be immovable. 



The locomotion of corals, therefore, is confined to the early period of their existence, is 

 more or less passive, and the organs concerned in it are the cilia. The cilia vary in length, 

 and their movement is vigorous ; their activity is increased by light, warmth, and a highly 

 aerated pure sea-water. 



The adhesion to the foreign substance occurs by means of the outer membrane : if the 

 base of the future corallum is to be small and pedunculate, the membranes at the base 

 grasp some irregularity of the surface of the stone or shell, as the case may be, or envelop 

 the body should it be small. As the hard parts are developed by the inner membranes, 

 they pass around or envelope the substance, and fix the coral permanently. Occasionally, 

 specimens are found with erosions at the base, as if they had sufi'ered a violent rupture 

 from the supporting substance and had continued to exist. 



When a broad and flat base occurs, either the membranes and the subsequently 

 developed sclerenchyma fill up the irregularities on the surface of the substance upon 

 which the polype has rested, or are attached to it by a secretion of the epitheca. When 

 corals rest on soft mud or sand, and become immersed, the tentacular disc appears just 

 above the surface, and the body of the coral is very generally found covered by the 

 epithecal membrane and its badly organised calcareous secretion. It is especially these 

 corals that have large lateral growths, large costae and processes ; and they may be broad 

 at the base, or quite the reverse. 



The epitheca acts as an anchor and as a sheathing to the coral. 



It has already been noticed, that the skeleton of the coral — its sclerenchyma — is 

 developed and nourished by the inner membrane; and the retreat of this membrane, as well 

 as the apparent death of all the hard pai'ts below its level, have been explained. It will 

 be found that the inner membrane permeates the hard tissues, that these are developed 

 as granules in its intercellular spaces, and that, as the granules become hard, close, and 

 solid, the nourishing influence of the membrane gradually ceases. In perforate corals the 

 membrane is always in contact with the reticulate sclerenchyma, and the interiors of 

 adjacent corallites are constantly in mutual relation. 



Considering the weight of many individual corals, and the tenuity of the soft parts, 

 this development of sclerenchyma is very wonderful. It must be remembered, that in 

 many large compound corals only the few upper lines of the corallites are really nourished 

 by the soft parts ; all the rest has been gradually developed and left by them. 



The density of the sclerenchyma difiers more in species than in individuals, and size 

 has nothing to do with it. As a rule, very quickly growing corals are less dense than 



