CCELENTEKATA : ACTINOZOA. 105 



which often meet so as to form ' transverse props extending 

 across the loculi like the bars of a grate, and termed " synap- 

 ticulae." : 



The Zoantharia Sclerodermata are divided into the four 

 following groups, founded upon the characters of the 

 corallum : 



1. Tabulate. 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 tabula?. 

 Corallum composed of compact, imperforate sclerenchyma. 



4. Tubulosa. Septa indicated by mere stria3 ; thecae pyri- 

 form, occasionally united by a basal coenenchyma. 



GEMMATION AND FISSION AMONGST CORALS. As regards the 

 modes in which the composite corals are produced the follow- 

 ing is a summary of Professor Greene's remarks upon this 

 subject. (See Coelenterata, p. 185 et seq.) The production of 

 the composite Actinozoa is effected either by gemmation or by 

 fission. In the former method three varieties have been dis- 

 tinguished, termed respectively ' basal,' * parietal,' and ' cali- 

 cular' gemmation. 



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

 rudimentary coanosarc, 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 coenosarc 

 remains soft, or deposits a coenenchyma ; 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 

 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. 2 7, a) . Fission in the Actinozoa differs 



