THE LOWER FORMS OF LIFE. 



115 



It is quite evident that after a time the first race of Corals can 

 extend no farther. They then die, leaving behind them a progeny 

 which cover the old Coral of their parents. This process is repeated 

 in some species indefinitely, each colony of course increasing the size 

 of the corallum. In others, as in the well-known " Brainstone Coral," 

 the size of the mass is limited. I have a fine specimen brought 

 from Bermuda by Monsignor Virtue, and kindly presented to me, 

 which is 54 inches in circumference and 30 inches from point to 

 point over the apex. This is among the largest specimens. 



I will now give two diagrammatic sectional figures of a corallite, 

 one of the common reef-builders, which will illustrate the mode by 

 which all sclero-dermic Corals are formed. 



Fig. 150. Longitudinal Section of Corallite. 

 (Sclero-dermic.) 



Fig. 151. Transverse Section of Corallite. 

 (Sclero-dermic.) 



a a, Calice or cup &, external wall or theca c, axis or columella (not always present) 

 d d, septa, some complete, some incomplete e e, interseptal dissepimenta //, tubular 

 dissepiments g g, loculi h h, pali. 



It must not, however, be forgotten that the figures are what I say, 

 only diagrams ; and further, that they represent hard parts laid 

 down by the inner skin or endoderm of the polyp, and consequently 

 the internal form of the polyp whose history we are inditing. I am 

 thus particular because I wish to be clear, and to illustrate this 

 somewhat complex subject in the simplest form of which it will 

 admit. 



A longitudinal section, therefore, of a corallite (Fig. 150) repre- 

 sents, as seen at b, an external wall or theca, and an axis or 

 columella (c), as it is termed, in the centre of the cup-like corallite, 

 which is not, however, always present. 



The figures (a, a, a, a) show the sectional half of the cup or calice 

 of the corallite. The lines (d, d) indicate the thin plates which are 



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