CCELENTERATA : ACTINOZOA. 



upper half of the coil covered by a uniform cylindrical bark, 

 regularly studded with retractile polypes." The lower portion 

 of the siliceous rope-like axis, which looks exactly like a skein 

 of threads of glass, is sunk in the sand at the bottom of the 

 sea. The upper portion of the Hyalomma is often occupied 

 by a cup - shaped sponge, called Carteria, which Dr Gray 

 believes to be a parasitic growth. By Professors Loven, 

 Perceval Wright, Wyville Thomson, and others, the sponge 

 Carteria is looked upon as the true artificer of the siliceous rope, 

 and the polypes are regarded as parasitic, and as referable to 

 Palythoa. This last view, by which Hyalonema would be 

 placed amongst the siliceous sponges, appears upon the whole 

 to be most probably the correct one. In this case there is no 

 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 

 coenosarc. 



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. 30, </) 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 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 

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

 furnished with styliform or spine-like processes growing from 



