CCELENTERATA : ACTINOZOA. 123 



The divisions of the sub-order are founded upon the nature 

 of the corallum, for the due comprehension of which it will 

 be necessary to consider some points in connection with these 

 structures somewhat more minutely. As already described, a 

 typical corallite consists of an outer wall or " theca," with a 

 cup or " calice " above, and divided below into numerous 

 chambers or " loculi " by vertical partitions or " septa." Often 

 the larger or " primary " septa coalesce centrally to form a 

 median calcareous rod or "columella." The chief additional 

 structures to be remarked are what are known as " tabulae," 

 and " dissepiments." The " tabulae" (fig. 52, d) are transverse 

 plates or floors running at right angles to the axis of the coral- 

 lite, and dividing the theca into so many horizontal compart- 

 ments or stories, each of which is vertically subdivided by the 

 septa, when these exist. As a rule, however, the septa are 

 absent when there are tabulas, though the two structures co- 

 exist in many extinct corals. The " dissepiments " are incom- 

 plete transverse plates, which, " growing from the sides of the 

 septa, interfere, to a greater or less extent, with the perfect 

 continuity of the loculi." — (Greene.) The septa, too, are often 

 furnished with styliform or spine-like processes growing from 

 their sides, which often meet so as to form " transverse props 

 extending across the loculi like the bars of a grate, and termed 

 'synapticulas.'" 



The Zoanfharia Sderodermata are divided into the four 

 following groups, founded upon the characters of the co- 

 rallum : — 



1. Tabulata.- — Septa rudimentary, or entirely absent ; tab- 

 ulae well developed, and dividing the visceral chamber into a 

 series of stories. 



2. Perforata. — Septa well developed ; dissepiments mdi- 

 mentary ; no tabulae. Corallum composed of porous scleren- 

 chyma. 



3. Aporosa. — Septa well developed, lamellar ; no tabulae. 

 Corallum composed of compact, imperforate sclerenchyma. 



4. Tubulosa. — Septa indicated by mere striae; thecae pyri- 

 form, occasionally united by a basal ccenenchyma. 



Gemmation and Fission amongst Corals. — As regards 

 the modes in which the composite corals are produced, the 

 following is a summary of Professor Greene's remarks upon 

 this subject. (See Caelenterata, p. 185 et seq.) The produc- 

 tion of the composite Adinozoa is effected either by gemma- 

 tion or by fission. In the former method three varieties have 

 been distinguished, termed respectively " basal," " parietal," 

 and " calicular" gemmation. 

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