COELENTEEATA 33 



presents a ring of ectoderm and a ring of endoderm, between 

 which there is a third supporting layer mutually secreted 

 between the adposed faces of the two layers. This layer, the 

 mesogloea, is not formed of cells. 



The Ectoderm is protective and nervous, and a closer study 

 of sections will show that the cells, though apparently all very 

 similar, are modified for various purposes. The layer is made 

 up mainly of cells, columnar in shape, but broader externally. 

 They form a columnar epithelium which is flattened on the 

 hypostome. Externally they define the general surface of 

 the body, and the exposed cytoplasm of each presents a 

 distinct ectoplasm. They gradually taper towards the 

 mesogloea, against which layer they expand into one, or some- 

 times two or three processes which are directed longitudinally. 

 Externally these cells are sensory and internally contractile ; 

 they are not merely protective, they are neuromuscular. 

 Certain cells, moreover, have become especially nervous. 

 Such will be seen near or applied to the mesogloea and 

 recognised by their small size, large nucleus, and the fine 

 processes which proceed from them. These processes are 

 nerve fibres, and are formed into a network coming into close 

 association with the contractile filaments of the epithelial 

 cells. They are thus associated with bringing about a con- 

 certed action in the contraction of the body. 



Generally distributed likewise, and only slightly different 

 from the general neuromuscular cells, are cells which are 

 glandular, and secrete a substance of the nature of mucin. 

 Practically all the cells of the basal disc are of this nature. 

 Between the epithelial cells at their bases are numbers of 

 small cells which are called interstitial cells. These may 

 replace all the others, are especially concerned with repro- 

 duction, and give rise to a third type of cell which is eminently 

 characteristic of the Coelenterata, the stinging cell. Stinging 

 cells are common on the tentacles, where they occur in groups 

 or batteries, and in the upper part of the column. 



The stinging cell or nettle cell nematoblast or cnidoblast 

 consists of a cell in the form of a cup within which is pro- 

 duced a capsule containing fluid. The external wall of the 

 capsule is infolded into the capsule as a long spirally coiled 



