104 MANUAL OP ZOOLOGY. 



so-called ' Glass-zoophytes,' the true nature and position of 

 which has been a subject of much controversy. By Dr. Gray 

 the Hyalonemadce are believed to be true Actinozoa, and 

 he defines them as follows : ' Social Zoanthoid polypes 

 secreting a central, siliceous, internal, axial coil for their 

 support. The 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 Hyalonema is 

 often occupied by a cup-shaped sponge, called Ccirteria, 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. 



SUB-OEDEE III. ZOANTHAEIA ScLEEODEEMATA. The members 

 of this sub-order include the great bulk of the coral-producing 

 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 actino- 

 soma 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. 27, d) are transverse plates 

 or floors running at right angles to the axis of the corallite, and 

 dividing the theca into so many horizontal compartments 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 coexist in many extinct 

 corals. The * dissepiments ' are incomplete 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 their sides, 



