I 34 MANUAL OF ZOOLOGY. 



Actinozoon, as far as is yet known, which possesses the power 

 of secreting a siliceous skeleton, in this respect presenting a 

 striking contrast to the Protozoa. 



SUB-ORDER III. ZOANTHARIA SCLERODERMATA. The mem- 

 bers of this sub-order include the great bulk of the coral-pro- 

 ducing or " coralligenous " zoophytes of recent seas. They 

 are defined by the possession of a sclerodermic corallum, the 

 parts of which are arranged in multiples of five or six. The 

 actinosoma may be simple, consisting of a single polype, or it 

 may be composite, consisting of several polypes united by a 

 ccenosarc. 



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. 46) are transverse 



Fig. 46. Columnaria. alveolata, showing the corallites partitioned off 

 into stories by tabulae. 



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 tabulae, 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 



