CCELENTERATA — CORALS 1 29 



derm bends upward just as in the formation of septa, except that 

 it is domed up instead of being ridged up. In the interior of a 

 coral there are usually present besides the septa, tabulae and 

 dissepiments. TabidcB are the transverse partitions extending 

 across the entire space within a coral from wall to wall, secreted 

 by the ectoderm of the base of the polyp like successive basal 

 plates. As the sides of the theca in some species grow higher 

 the polyp cannot occupy the entire interior and hence draws 

 itself upwards, and wherever it rests for a time a tabula is built 

 beneath it. These periods of quiescence may follow the expend- 

 iture of energy at times of reproduction, either sexual or asexual. 

 Dissepiments are the horizontal or oblique plates connecting 

 adjoining septa, formed in a manner similar to tabulae, except 

 that the base of the polyp is withdrawn in sections, not as a dis- 

 tinct whole, nor at such regular intervals. 



The nettle-cells (nematocysts) contain barbed threads (some 

 about one-fortieth of an inch long) which can pierce by their 

 sudden unrolling a man's calloused foot ; millions occur on the 

 tentacles. Each one is used but once, but new ones are con- 

 tinually being formed. The polyp can reproduce lost parts 

 and a small piece of the base can reproduce the w^hole animal. 



Reproduction is sexual or asexual. In sexual reproduction 

 the reproductive elements are formed in the endoderm of the 

 mesenteries ; there is no medusa stage. The new animal 

 resulting from this method of reproduction is a single individual. 



In asexual reproduction the process of budding or of division 

 forms from this single individual a colony of various shapes 

 and often of great size. In the Alcyonaria buds are always 

 formed on tubular outgrowths of the polyps, the solenia, 

 never directly from the polyps {e.g. Tubipora, Syringopora) . 

 In the Zoantharia buds arise directly from the polvp 

 or from the coenosarc. Except for a few forms, this 

 sub-class (Zoantharia) is divided into the perforate and 

 the imperforate orders. In the perforate corals (as Madrepora) 

 the spongy tissue (coenenchyme) is perforated by many canals 



