11 8 MANUAL OF ZOOLQGY. 



common Actinia. The tentacles are tubular prolongations of 

 the ectoderm and endoderm, containing diverticula from 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. As the stomach 

 is considerably shorter that 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 

 downward and inwards, are ultimately attached to the centre 

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

 lamellse 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 Adinim 

 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 Actinidce, 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 Actinios 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^. — In this family there 's no corallum, 

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

 base (fig. 31, b). Jtyantkusisia all essential respects identi- 

 cal 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. 



