STRUCTURE OF A SEA-ANEMONE. 139 



communication. The mouth is usually a longitudinal slit, 

 and its two corners are often richly ciliated and continued 

 into grooves (" siphonoglyphes ") down each side of the 

 gullet. Along these two grooves, and by these two corners, 

 food-particles usually pass in ; but in some it seems that one 

 side is an incurrent, the other an excurrent channel. Occasion- 

 ally, only one corner of the mouth and side of the gullet 

 is thus modified. The gullet often extends far down into 

 the cavity of the anemone, and admits of a certain amount of 

 extrusion. The mesenteries bear («) mesenteric filaments ; 

 {b) retractor muscles ; (c) ridges of reproductive cells, 

 almost always either ova or spermatozoa, rarely both ; and 

 (d) in some cases offensive threads (acontia), rich in stinging- 

 cells, and extrusible through holes in the body-wall. The 

 mesenteric filaments seem to be closely applied to the food 

 and secrete digestive juice. Sea-anemones have no sense- 

 organs ; the sapphire-beads, which are so well seen at the 

 bases of the outermost tentacles of the common Actinia 

 mesembryanthemum, are really batteries of stinging-cells. 

 The nervous system is uncentralised, and consists of super- 

 ficial sensory cells connected with a plexus of sub-epithelial 

 ganglion-cells. 



The Layers of the Body. — The ectoderm which clothes the exterior is 

 continued down the inside of the gullet. The endoderm lines the 

 whole of the internal cavity, including mesenteries and tentacles. The 

 mesoglosa is a supporting plate between these two layers, and forms a 

 basis for their cells. 



The ectoderm consists of ciliated, sensory, stinging, and glandular 

 cells, and also of sub-epithelial muscle and ganglion cells based on the 

 mesogloea, but mainly restricted to the circumoral region. 



The endoderm consists mainly of flagellate cells, with muscle-fibres 

 at their roots. These form the main muscle-bands of the wall, the 

 mesenteries, and the gullet. Nor are glandular and even sensory cells 

 wanting from the endoderm. 



The Mesenteries. — In Sea-Anemones and nearly related Anthozoa 

 twelve primary mesenteries are first formed. These are grouped in 

 pairs, and the cavity between the members of a pair is called intra- 

 septal, in contrast to the inter-septal cavities between adjacent pairs. 

 In these inter-septal chambers other mesenteries afterwards appear in 

 pairs. Two pairs of mesenteries, however, differ from all the rest, 

 those, namely, which are attached to each corner of the mouth and to 

 the corresponding grooves of the gullet. These two pairs of mesen- 

 teries are called " directive," and they divide the animal into bilaterally 

 symmetrical halves. Anatomically, a pair of directive mesenteries 

 differ from the other paired mesenteries, because the retractor muscles 



