Il8 MANUAL OF ZOOLOGY. 



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

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

 ' synapticulae.'" 



The Zoantharia Sderodermata are divided into the four 

 following groups, founded upon the characters of the co- 

 rallum : 



1. Tabulata. Septa rudimentary, or entirely absent ; 

 tabulae well developed, and dividing the visceral chamber into 

 a series of stories. 



2. Perforata. Septa well developed ; dissepiments rudi- 

 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 Ccelenterata, p. 185 et seq.} The produc- 

 tion of the composite Actinozoa 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. 



In basal gemmation the mode of increase is by means of a 

 rudimentary ccenosarc, which is put forth by the original 

 polype, and from which the young polype-buds are produced. 

 It " affords very different products according as the ccenosarc 

 remains soft, or deposits a ccenenchyma ; appears under the 

 form of stolons, or of stouter connecting stems ; or even 

 spreads out in several directions as a continuous horizontal 

 expansion ;" in which last case the youngest polypes are, of 

 course, those nearest to the periphery of the mass. 



The parietal mode of gemmation is the commonest, and it 

 gives rise chiefly to dendroid, or tree-like, corals. In this 

 method the buds are produced from the sides of the original 

 polype, and they often repeat the process indefinitely. 



Calicular gemmation is not known to occur in any recent 

 coral, but it was a common mode of increase amongst extinct 

 forms. In this method " the primitive polype sends up from 

 its oral disc two or more similar buds ; these, in their turn, 

 produce other young polypes, and thus the process is repeated 

 until an inverted pyramidal mass of considerable size is pro- 

 duced, all the parts of which rest upon the narrow base of the 

 first budding polype (fig. 30, a). Fission in the Actinozoa differs 



