156 MANUAL OF ZOOLOGY. 



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" mes- 

 enteries, according to their breadth. The reproductive organs 

 (fig. 35) are in the form of reddish bands, which contain ova 

 and spermatozoa, and are situated on the faces of the mesen- 

 teries. Most of the Actinice are dioecious that is to say, the 

 same individual does not develop both ova and spermatozoa ; 

 but some forms are monoecious. The free edges of the mes- 

 enteries below the stomach are thickened, and constitute 

 puckered and convoluted cords ("craspeda"), which are richly 

 furnished with thread-cells. Attached also to the free edges of 

 the mesenteries are sometimes found long thread-like filaments 

 likewise crowded with thread-cells. These peculiar structures 

 (" acontia"} appear to be organs of offence and defence, as 

 they can, on irritation, be rapidly shot forth from the mouth, 

 as well as from certain minute orifices in the body-wall (" cin- 

 clides ") which appear to be specially intended for their emis- 

 sion. As regards their nervous system, nerve-cells and anas- 

 tomosing nerve-fibres are stated to be present in the base 

 (Martin Duncan), and may exist in other parts of the Sea- 

 anemones, whilst the pigment - masses at the bases of the 

 tentacles in some forms appear undoubtedly to be rudimentary 

 organs of vision. 



The embryo of the Actinia is a free-swimming ciliated body, 

 at first rounded, but afterwards somewhat ovate. The rudi- 

 mentary 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 two 

 in number, but are rapidly increased to six. 



FAMILY II. ILYANTHID^E. In this family there is no corallum, 

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

 base. Ilyanthus itself (fig. 71, E) is in all essential respects 

 identical with the ordinary Actinice, but it is of a pointed or 

 conical shape, the base being much attenuated, and it leads a 

 free existence. Arachnactis (fig. 70, b) is also free, and accord- 

 ing to Professor E. Forbes, it can not only swim like a jelly- 

 fish, but "it can convert its posterior extremity into a suctorial 

 disc, and fix itself to bodies in the manner of an Actinia" It 

 is by no means certain, 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 (perhaps 

 of Edwardsia). 



