IV STRUCTURE 293 



The essential feature in the arrangement of the cells is 

 that they are disposed in two layers round the central diges- 

 tive cavity or enteron (a, ent. caii) and the cavities of tenta- 

 cles (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 membrane, the mesoglcea, or sup- 

 porting lamella {msgl). A transverse section (b) shows that 

 the cells in both layers are arranged radially. 



Thus Hydra is a two-layered 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 complicated 

 than the foregoing brief descripton would indicate. 



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

 obvious (13, ed, 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 be- 

 tween their larger companions and are distinguished as 

 interstitial cells. 



The inner ends of the large ectoderm cells are continued 

 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 



