CCELEffTERATA. 31 



Subclass I. ZOANTHAEIA. 



POLYACTINIA. 



Polypes with simple or occasionally branched tentacles, six, or 

 a multiple of six. Corallum, when present, mostly sclerodermic, 

 more rarely sclerobasic. 



The corallum is a hard, mostly calcareous substance [calcium 

 carbonate], secreted externally in the sclerobasic corals, and inter- 

 nally in the sclerodermatous ; in the Malacodermata it is, when 

 present, disseminated in the form of small spicules [sole rites]. An 

 individual [persona] of the compound corallum is known as a 

 " corallite ;" the outer wall forms the " theca," the upper part of 

 which is the cup or calicle ; below it is often divided radially by 

 distinct vertical septa [mesenteries] united in the centre to the 

 "columella." Sometimes the thecse are divided by horizontal 

 plates [tabulae]. " Dissepiments " are incomplete plates growing 

 from the sides of the septa. The " ccenenchyma " is the calcareous 

 covering uniting the individual corallites together ; it is secreted 

 by the'coenosarc, with which it may be said to correspond. 



Milne-Edwards divides the Zoautharia into three groups : 



Corallum either absent or rudimentary MALACODERMATA. 



Corallum present 



Corallum internal, calcareous SCLERODERMATA. 



Corallum external, not calcareous SCLEROBASICA. 



Order I. MALACODERMATA. (Sea-anemones.) 

 HELIANTHOIDA. ACTINIARIA. HEXACORALLA. 



Corallum absent, or represented by a few spicules. Tentacles nu- 

 merous, simple. Polypes rarely aggregated. Sexes mostly distinct. 



In the Zoanthidse only are the polypes united by a common 

 creeping stem. The majority, owing to their muscular base, enjoy 

 some power of locomotion, although generally adherent to some 

 foreign body. The Ilyanthidae have no adherent base ; and 

 Arachnactis is a free-swimming organism : it is, however, possibly 

 an immature form. 



The tentacles are generally disposed in two or more series. 

 These are successively developed from within outwards, in multi- 

 ples of six ; but one or more tentacles are sometimes abortive. 

 They are often perforate at the end. In Sagartia bellis, a common 

 species, there are six rows, the inner minute, and altogether they 

 amount to about 500. [Gosse.] 



