xxii MINUTE STRUCTURE 225 



it into a series of extremely thin sections and examining 

 them under a high power. The appearance presented by a 

 vertical section through the long axis of the body is shown 

 in Fig. 51. 



The whole animal is seen to be built up of cells, each 

 consisting of protoplasm with a large nucleus (p, nu\ and 

 with or without vacuoles. As in the case of most animal 

 cells, there is no cell-wall. Hydra is therefore a solid aggre- 

 gate ; but the way in which its constituent cells are arranged 

 is highly characteristic and distinguishes it at once from a 

 plant. 



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. aw) and the cavities of 

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

 throughout of an outer layer of cells, the tctoderm (ect\ and 

 of an inner layer, the endoderm (end\ which bounds the 

 enteric cavity. Between the two layers is a delicate trans- 

 parent membrane, the mesoglcsa, or supporting lamella (msgl). 

 A transverse section shows that the cells in both layers are 

 arranged radially (B). 



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 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 



Q 



