THE ANTHOZOA 69 



in which the calicles lie is in its origin a thecal structure, 

 and that it is impossible to say where the theca of one corallite 

 ends and that of another begins. In aporose corals, on the other 

 hand, the theca is a well-defined structure, and the calcareous 

 tissue in which the corallites are imbedded is a secondary deposit 

 of entirely different origin. 



In the formation of colonies by division a distinction must be 

 made between equal and unequal division, though the two processes 

 merge into one another. The process of equal division is well 

 illustrated by Mussa (Fig. XXX. 5, 6). The zooid, previously 

 subcircular in section, becomes elongated in the direction of the 

 long axis of the mouth, and at the same time the tentacles, mesen- 

 teries, and septa increase in number. A constriction, at right 

 angles to the long axis of the mouth, involves first the mouth, 

 then the peristome, and finally the calyx itself, so that the zooid 

 and its corallite, previously single, becomes divided into two. The 

 part played by the septa and theca will be best understood by a 

 study of Fig. XXX. 5. After division the two corallites grow 

 upwards ; at first their zooids are united by a bridge of soft tissue 

 or edge-zone, but as they grow further and further apart this con- 

 tinuity is broken, each corallite is clothed externally to a greater 

 or less extent by its proper edge-zone, and, as the interseptal loculi 

 become closed below by dissepiments, all organic connection 

 between the two zooids is eventually lost, though the corallites 

 remain attached to one another. There are, however, forms 

 not far removed from Mussa in which the corallites are closely 

 apposed after division, the continuity of the edge-zone is not 

 broken, and growth leads to the formation of a coenosarc which, 

 as in the case of colonies produced by gemmation, gives rise to a 

 coenenchyme filling up the spaces between the corallites. The 

 complex Maeandrine corals are produced by incomplete division 

 which involves the mouth, and to some extent the peristome, but 

 does not extend to the calyx. Repetition of this incomplete 

 division gives rise to long Maeandrine channels, each containing 

 numerous zooid mouths. 



Unequal division may be studied in Lophohelia j)rolifera and 

 allied forms, and the pi'ocess is illustrated in Fig. XXX. 3, 4. 

 Instead of the whole calyx undergoing division, a small portion of 

 it is constricted off to form a young zooid which, in its earliest 

 stage, looks like a bud on the margin of the calyx. Reference to 

 3 shows, however, that the process of unequal division differs 

 from that of gemmation in that, in the former, the theca, septa, 

 and columella of the young zooid are directly formed from corre- 

 sponding structures in the parent. As growth proceeds, the smaller 

 or daughter calyx becomes more and more separate from the 

 larger or parent calyx, and eventually it looks like a lateral bud 



