ACTINOZOA. 



245 



nature. In many instances the entire corallum of a Madreporarian 

 consists of the ordinary fibrous sclerenchyma just alluded to. It is 

 not unusual, however, to find that in progress of growth the original 

 skeleton becomes thickened, and its internal cavities more or less 

 restricted, by a deposition of carbonate of lime of secondary origin. 

 This secondarily-formed calcareous tissue has been termed " stereo- 

 plasma" by Lindstrom, and it is often of lighter colour than the 

 sclerenchyma of the original skeleton (fig. 125, b), or is otherwise 

 distinguishable from the latter. 



An ordinary sclerodermic coral may consist of a single cuplike 

 structure corresponding with a single polype, or of several such, 



Fig. 125. — A, Portion of a tangential section of the corallum of the recent StyloftJwra palinata, 

 enlarged, showing the fibro-crystalline structure of the sclerenchyma. b, A few corallites of 

 Pachypora Nicholsoni, Freeh, transversely divided and considerably magnified, showing the 

 primordial wall (7') and the dense lining of secondary " stereoplasma" (s). From the Middle 

 Devonian of Gerolstein. (Original.) 



springing directly from one another, or united by a common cal- 

 careous tissue (" ccenenchyma ") corresponding with the ccenosarc 

 of the colony. 



A typical simple sclerodermic corallum (fig. 126) is secreted by a 

 single polype, and its structure presents an obvious correspondence 

 with that of the animal which produces it. It is generally more or 

 less conical in shape, or sometimes discoid, and consists of an outer 

 wall and included space. The wall corresponds with the lower part 

 of the column of the polype, and is known as the " theca." It may 

 be very imperfect, or may be strengthened by a secondary calcareous 

 investment ("epitheca"). The theca encloses a space which cor- 

 responds with the lower part of the body-cavity of the polype, and 

 is known as the " visceral chamber." Superiorly the theca termi- 

 nates in a shallower or deeper, cup-shaped depression, which contains 

 the oesophagus of the polype, and is known as the "calice." Below 



