298 HYDRA CHAP, v 



The essential feature in the arrangement of the cells 

 is that they are disposed in two layers round the central 

 digestive cavity or enteron (A, ent. cav) and the cavities 

 of the tentacles (ent. cav'). So that the wall of the body 

 is formed throughout of an outer layer of cells, the 

 ectoderm (ect), and of an inner layer, the endoderm (end], 

 which bounds the enteric cavity (compare p. 202). 

 Between the two layers is a delicate transparent mem- 

 brane, the mesoglcea, or supporting lamella (msgl). A 

 transverse section (B) shows that the cells in both 

 layers are arranged radially. 



Thus Hydra is a two-layer or diploblastic animal, and 

 may be compared to a chimney built of two layers of 

 radially arranged bricks with a space between the layers 

 filled with mortar or concrete. 



Accurate examination of thin sections, and of specimens 

 teased out or torn into minute fragments with needles, 

 shows that the structure is really much more compli- 

 cated than the foregoing brief description would indicate. 



The ectoderm-cells are of two kinds. The first and 

 most obvious (B, ect, and C) are large cells of a conical 

 form, the bases of the cones being external, their apices 

 internal. Spaces are necessarily left between their 

 inner or narrow ends, and these are filled up with the 

 second kind of cells (int. c), small rounded bodies which 

 lie closely packed between their larger companions and 

 are distinguished as interstitial cells. 



The inner ends of the large ectoderm-cells are con- 

 tinued into narrow, pointed prolongations (C, m. pr) 

 placed at right angles to the cells themselves and 

 parallel to the long axis of the body. There is thus a 

 layer of these longitudinally-arranged muscle-processes 

 lying immediately external to the mesoglcea (B, m. pr). 

 They appear to possess, like the axial fibre of Vorticella 



