148 CCELENTERA. 



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 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 chief muscle bands of the wall, the mesen- 

 teries, and the gullet. Nor are glandular and even sensory cells wanting 

 in 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 the two cornels of the mouth and to the 

 corresponding grooves of the gullet. These two pairs of mesenteries 

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

 metrical 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 im- 

 portance in classifying Anthozoa. It is possible that the mesenteries 

 are homologous with the teeniols 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. In 

 some cases the ovum segments into a solid morula ; this becomes a 

 free planula, in which a cylindrical depression at one pole forms 

 the mouth and gullet. 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. 

 According to Gcette, the development is in essentials the same as that 

 of the Hydra-tuba. 



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 the related forms. This contrast is not very satis- 

 factory, but it rests on such distinctions as the following : — 



