CCELENTERATA : ACT1NOZOA. 113 



of the base. Besides the primary mesenteries, there are other 

 lamellae which also arise from the body-wall, but which do 

 not reach so far as the outer surface of the stomach, and are 

 called " secondary " and " tertiary " mesenteries, according to 

 their breadth. The reproductive organs are in the form of 

 reddish bands, which contain ova and spermatozoa, and are 

 situated on the faces of the mesenteries. Most of the Actinia 

 are dioecious that is to say, the same individual does not 

 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 which is not yet understood. It is believed, how- 

 ever, that the apertures, termed " cinclides," in the column- 

 walls of some of the Actinida, are for the emission of the 

 craspeda. No traces of a nervous system have as yet 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. ILYANTHID/E. In this family there is no corallum, 

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

 base (fig. 29, b\ Ilyanthus is in all essential respects identi- 

 cal with the ordinary Actinia, 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. 

 Arachnactis is certainly free, and, according to Professor E. 

 Forbes, it can not only swim like a jelly-fish, but " it can con- 

 vert its posterior extremity into a suctorial disc, and fix itself to 

 bodies in the manner of an Actinia" It is by no means cer- 

 tain, however, that Arachnactis is a mature form, and there is 

 some reason to suppose that it is merely the young stage of 

 some at present unknown Actinozoon. 



FAMILY 3. ZOANTHID^:. In the Zoanthida 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 Actinia, but they are united together 

 at their bases by a thin fleshy ccenosarc. 



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. 



VOL. I. H 



