132 MANUAL OF ZOOLOGY. 



related in the most obvious manner to the soft structures 

 of the animal by which they are secreted. Thus, the 

 "theca" clearly corresponds to the "column-wall," or the 

 general wall of the body ; the " columella," when present, cor- 

 responds to " that part>-of the enderon which forms the floor of 

 the somatic cavity below the digestive sac ; " whilst the " septa " 

 correspond to the "mesenteries," and, like them, are called 

 "primary" and " secondary " according as they reach the colu- 

 mella or fall short of it. When there are several corallites, the 

 bond of union between them, the " coenenchyma," is secreted by 

 the " coenosarc," to which it corresponds. In many Actinozoa, 

 however, the sclerodermic corallum is not present in the typical 

 form above described, but simply in the form of calcareous 

 spicules or nodules scattered through the tissues of the animal. 

 There are, also, members of the class in which both a scleroder- 

 mic and a sclerobasic corallum are present, the latter constituting 

 the main skeleton, whilst the former is represented by scattered 

 spicules. The coral tissue itself is known as " sclerenchyma," 

 and it varies considerably in texture, being sometimes extremely 

 compact, and at other times very loosely put together. 



From what has been said it will be seen that a sclerobasic 

 corallum can easily be distinguished from a schlerodermic by 



Fig. 44. Sclerodermic and Sclerobasic Corals, a Portion of branch of Deudrophyllia, 

 nigrescens, a compound sclerodermic coral (after Dana) ; b Longitudinal section of 

 Isis hippuris, a sclerobasic coral, exhibiting the external bark or coenosarc, with 

 its embedded polypes, supported by the internal axis or skeleton (after Jones). 



inspection ; the former (fig. 44, b) being usually more or less 

 smooth, and being invariably devoid of the cups or receptacles 



