SOME ANIMAL ARCHITECTS. 137 



appear when the living matter is washed away. But in the 

 second variety of coral-structure, well exemplified by the 

 Carophyllia and the great reef-building corals (Fig. 14), the 

 coral-substance is outside the living parts, each little polype 

 being contained within a cell which it has secreted and 

 formed. This latter mode of growth produces the massive 

 solid corals, on the presence and increase of which the for- 

 mation of reefs depends ; the more delicate and branching 

 species being formed after the type of the red coral and its 

 neighbours. That lime is the chief element represented in 

 the coral-substance may be readily inferred from the pre- 

 ceding remarks. A few corals, however, exhibit a com- 

 position in which lime plays an altogether secondary part. 

 Thus the Is is or mare's-tail coral (Fig. 13) of the Indian 

 Ocean and elsewhere, consists of alternate joints of horny 

 and limy matter ; whilst in another group, represented by the 

 Gorgonias or " sea-fans," the coral is entirely composed of 

 horny material. The essential details comprised in the 

 general history of the coral-polypes may be briefly sum- 

 marised by way of introduction to the investigation of their 

 actual work in reef-formation, by asserting each coral-animal 

 to be in all essential details of structure a sea-anemone ; and 

 by the further statements, that the coral-polypes differ from 

 the anemones in respect of their ability to form an internal 

 or external skeleton usually consisting of limy matter, and 

 that they increase indefinitely by a process of budding or of 

 division, and thus give rise to connected colonies. Bearing 

 these details in mind, the further history of the operations 

 of these animals will be readily understood. 



Two important points in the life of the coral-polypes 

 demand attention by way of introduction to the general 

 history of their architectural operations. Like all other 

 living beings, the coral-animals require certain special con- 

 ditions as those of their normal existence. In the case 

 before us, the two conditions demanded are a certain tem- 

 perature and a certain depth of sea j these conditions con- 

 stituting the environments, as it were, of coral life. The 



