CCELENTERATA : ACTINOZOA. 101 



develop both ova and spermatozoa. Along the free margins 

 of the mesenteries there also occur certain singular, convoluted 

 cords, charged with thread-cells, and termed ' craspeda,' the 

 function of winch is not yet understood. It is believed, how- 

 ever, that the apertures, termed * ciiiclides,' in the column- 

 walls of some of the Actinidce are for the emission of the 

 craspeda. No traces of a nervous system have as yet been 

 been proved to exist in any Actinia. 



The embryo of the Actinia is a free- swimming ciliated 

 body, at first rounded, but afterwards somewhat ovate. The 

 rudimentary mouth is soon marked out by a depression at the 

 larger extremity ; thread-cells appear as a layer in the ecto- 

 derm ; a fold is prolonged inwards from the mouth to form 

 the digestive sac ; and the primitive tentacles are at first 

 either five or six in number, but usually double themselves 

 rapidly. 



FAMILY 2. ILYANTHIDJ:. In this family there is no corallum, 

 and the polypes are single and free, with a rounded or tapering 

 base. Ilyanthus is in all essential respects identical with the 

 ordinary Actinice, but it is of a pointed or conical shape, the 

 base being much attenuated, though whether its habit of life 

 is free, or not, is a matter of some uncertainty. 



FAMILY 3. ZOANTHID^S. In the Zoanthidce there is a spi- 

 cular corallum, and the polypes are attached by a fleshy or 

 coriaceous base or ccenosarc. In Zoanthus the separate polypes 

 closely resemble small Actinice, but they are united together 

 at their bases by a thin fleshy coenosarc. 



SUB-ORDER II. ZOANTHARIA SCLEROBASICA. The members of 

 this sub-order are always composite ; and always possess a 

 corallum, but this is ' sclerobasic,' and there are no spicular 

 tissue-secretions. 



It appears advisable to explain here what is understood by 

 the terms ' sclerobasic ' and ' sclerodermic,' as applied to 

 corals. The ' corallum ' is the term which is applied to the 

 hard structures deposited by the tissues of any Actinozoon, 

 many of which are so familiarly known as * corals.' Usually 

 the corallum is composed of carbonate of lime ; but it may be 

 corneous, or partly corneous and partly calcareous ; whilst in 

 one family it appears to be siliceous. Whatever their compo- 

 sition may be, all coralla may be divided into two sections, 

 termed respectively ' sclerobasic ' and ' sclerodermic,' which 

 must be carefully distinguished from one another. The 

 ' sclerobasic ' corallum, of which the red coral of commerce 

 may be taken as the type, is in reality an exoskeleton, some- 

 what analogous to the shell of a Crustacean, being a true 

 tegumentary secretion. At the same time, it is not a shell or 



