I30 AN INTRODUCTION TO THE STUDY OF FOSSILS 



permanently connecting the digestive cavities of all the polyps 

 of the colony ; the coenenchyme is deposited both around these 

 canals and by the superficial coenosarc. In the imperforate 

 corals (as Astrangia) there are a few canals between the surface 

 of the coenenchyme and the coenosarc, not penetrating the 

 coenenchyme; these connect the polyps over the edge of the 

 calices, not directly through the cups. The coenosarc is nour- 

 ished by these canals and secretes the coenenchyme. 



In the Zoantharia, asexual reproduction takes place also by 

 equal or unequal division, that is, the polyp divides into two 

 or more individuals, each taking a portion of the mouth and some 

 of the tentacles, mesenteries, etc. This division usually takes 

 place by the sides of the mouth growing inward, thus gradually 

 constricting the original mouth. There thus result two mouths 

 in place of one, each surrounded by its own circle of tentacles. 

 Usually the digestive cavity also divides completely so that in 

 each separate polyp the mesenteries and consequently the septa 

 radiate in all directions from a separate center. The incomplete 

 division is illustrated by the " brain-coral " (Meandrina) ; 

 here the mouth divides completely but not the digestive cavity 

 and consequently not the calyx ; a repetition of such division 

 gives rise to the long, calcareous furrows, characteristic of 

 Meandrina, each surmounted by many polyp mouths. 



Distribution takes place mainly in the larval stage when the 

 animal is free-swimming. All forms are marine. They occur 

 from sea-level to a depth of three and a fourth miles, and 

 therefore at all temperatures. The deep-sea corals are solitary 

 forms. Compound corals are very abundant on coral-reefs. 

 The condition for the reef-building corals is that the tempera- 

 ture be not lower than 68° F., the water usually not deeper than 

 150 feet, and that there be but little sediment in the water. 



In the formation of coral-reefs corals enter to a much less 

 degree than was formerly supposed ; in at least many reefs lime- 

 secreting plants are the chief agent. (See p. 2>^.) 



Anthozoa are known from the Cambrian to the present. 



