CCELENTERATA : ACTINOZOA. I 29 



the somatic chambers, and sometimes having apertures at their 

 free extremities. The mouth leads directly into the stomach, 

 which is a wide membranous tube, opening by a large aper- 

 ture into the general body-cavity below, and extending about 

 half-way between the mouth and the base. The wide space 

 between the stomach and column-wall is subdivided into a 

 number of compartments by radiating vertical lamellae, termed 

 the " primary mesenteries/' arising on the one hand from the 

 inner surface of the body-wall, and attached on the other to 

 the external surface of the stomach. In reality, the mesenteries 

 are arranged in pairs, the chamber between each pair opening 

 above into the cavity of a tentacle. As the stomach is 

 considerably shorter than the column, it follows that the inner 

 edges of the primary mesenteries below the stomach are free ; 

 and these free edges, curving at first outwards and then down- 

 wards and inwards, are ultimately attached to the centre 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, however, that 

 the apertures, termed " cinclides," in the column-walls of some 

 of the Actinidce, are for the emission of the craspeda. No 

 traces of a nervous system, except the previously mentioned 

 " optic nerves," 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 II. ILYANTHID^E. In this family there is no corallum, 

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

 base (fig. 41, 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. 



* I 



