152 CCELENTERA. 



the bases of the outermost tentacles of the common Actinia 

 mesembryanthemum, are batteries of stinging cells. The 

 nervous system is uncentralised, and consists of superficial 

 sensory cells connected with a plexus of sub-epithelial 

 ganglion cells. 



7^he 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 meso- 

 glcea 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 

 mesoglcea, 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 mesen- 

 teries, 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 correspond- 

 ing grooves of the gullet. These two pairs of mesenteries are called 

 " directive," and they divide the animal into bilaterally symmetrical 

 halves. Anatomically, a pair of directive mesenteries differs from the 

 other paired mesenteries, because the retractor muscles which extend in 

 a vertical ridge along them, are turned away from one another, and run 

 on the inter-septal surfaces, whereas in the other mesenteries the 

 retractor muscles run on the intra-septal surface, those of a pair facing 

 one another. The arrangement of these muscles is of great importance 

 in classifying Anthozoa. It is possible that the mesenteries are 

 homologous with the taeniolge of jelly fish, and the mesenteric with the 

 gastric filaments. 



From the above description, it will be noticed that the funda- 

 mental radial symmetry of the Ccelentera has here become profoundly 

 modified. 



Development. Comparatively little is known in regard to the early 

 stages of development in sea .anemones. From the fertilised ovum, a 

 blastosphere may result which by invagination becomes a gastrula. Or 

 the two layers may be established by a process known as delamination, 

 in which a single layer of cells is divided into an inner endodermic and 

 an outer ectodermic layer. 



Related Forms. The sea anemones are classified in the sub-class 

 Anthozoa or Actinozoa, and along with many corals are distinguished as 

 Zoantharia or Hexacoralla from the Alcyonaria or Octocoralla, like 

 Alcyonium and related corals. This contrast is not perfectly satisfactory, 

 but it rests on such distinctions as the following : 



