II ACTINOZOA 113 



calcium carbonate and bulky in amount, forms a characteristic external 

 skeleton — coral. 



These zoantharian corals show a wonderful variety of form and size 

 and complexity. In the simplest type there is first laid down a thin 

 flat plate of calcium carbonate between the base of the polyp and the 

 solid substance, rock or shell, on which it rests (Fig. 51, tJi). Along 

 radiating lines, alternating with the mesenteries, the secretion becomes 

 more active so that ridges are formed which increase in height and become 

 thin vertical plates — the septa (cf. Figs. 52 and 53). A circular ridge is 

 formed connecting the septa near their outer ends, and this also increasing 

 in height forms the rim of a cup or theca in which the polyp rests 

 (cf. Fig. 52, A). Very often a mound of calcium carbonate is deposited 

 in the centre from which the septa radiate, and this increasing in height 

 becomes the columella. It is important to 

 bear clearly in mind that all this calcium 

 carbonate is laid down by the outer surface 

 of the ectoderm of the base of the polyp, so 

 that it is strictly speaking entirely outside 

 the living substance ; it is an external skeleton 

 or exoskeleton, although its various parts are 



closely ensheathed by the polyp floor which "^^^^^^^^^^^fi ^^ 



is pushed upwards as they increase in height. 



The simplest type of coral is a simple cup 



. . . A youngCoral-polyp {.4sirot(ics), 



or theca with radiatmg septa (Fig. 52, A) (After Lacaze-Duthiers.) th, Cal- 



but there exist many complications of this '^^'^^^ ^1^'! i"'^"^ ""' '°"' 



•' i mencement of the theca. 



simple type, due for the most part to 



peculiarities in processes of asexual reproductidn. Coral polyps, unlike 

 ordinary anemones, very usually form communities or colonies by 

 processes of budding or fission. By budding a tree-like colony may 

 be built up of numerous conical thecae, each containing an isolated 

 polyp, as in the case of Lophohelia (Fig. 52, B), masses of which 

 are sometimes drawn up on fishermen's long lines off our western 

 and northern coasts. In other cases the polyps do not become 

 completely separated but remain in continuity. In this case the 

 layer of living tissue between the polyps (coenenchyme) goes on 

 secreting calcium carbonate on its basal surface and consequently the 

 individual thecae instead of being quite separate are connected 

 together by an intervening solid mass (Fig. 52, C). Again in other 

 cases a process of imperfect fission takes place. The polyp becomes 

 much drawn out so that it is band-shaped instead of circular as 

 seen from above, but there is no attempt at division into separate 



I 



