138 NATURAL HTSTOEY OF 



anemones, the different points of structure may be 

 more fully dealt with. 



The structure of the body wall and septa reveals the 

 presence of a threefold layer of cells, forming respec- 

 tively the ectoderm (ec), mesoderm [mes), and endo- 

 derm (e«). (See Plate XI. fig. 4.) The mesoderm may 

 be taken as the groundwork of the structure, and 

 extends throughout the body wall, oesophagus, and 

 septa. The ectoderm extends along the outside of 

 the body wall and the inside of the oesophagus. The 

 endoderm covers the whole interior of the body wall 

 and tentacles, and lines the septa on both sides. 



The ectoderm, according to the most recent observers, 

 Heider and Hertwig, contains three principal layers of 

 cells — epithelial, nervous [n), and muscular [m). The 

 epithelial cells are found to possess well-defined cha- 

 racteristics, and to be capable of subdivision into four 

 layers — ciliated, stinging, glandular, and sensory. The 

 ciliated cells bear a bunch of cilia or a simple flagellum, 

 the latter form, however, occurring oftener in the epi- 

 thelial cells of the endoderm. The form of the cilia is 

 liable to modification, those on the tentacles being 

 modified so as probably to serve tactile purposes. The 

 stinging or urticating cells, or nematocysts (c), contain 

 the stinging threads. The glandular {d) cells contain 

 glandular secretions. The sensory cells are fine and 

 filamentous, and give off nerve threads. All the 

 epithelial cells, except the glandular ones, give ofE 

 processes, either in the form of cilia or tactile bristles. 

 The stinging and sensory cells are most numerous in 

 the epithelium of the oral disc and tentacles, the ciliated 

 and glandular cells increasing on the column and base. 

 The nervous layer (?i) of the ectoderm is found at the 



