PHYLUM COELENTERATA 85 



its numerous darkly stained nematocysts, the line of mesogloea, and the elongated 

 irregular highly vacuolated entoderm cells. In the region of the hypos tome 

 note the granular appearance of the entoderm, indicating glandular functions, as 

 it here secretes mucus to aid in swallowing food; observe also the folds in this 

 region, permitting great distension of the mouth. On some of the sections the 

 continuation of the gastrovascular cavity into the tentacles can probably be 

 observed. Make a low-power diagrammatic drawing of the section, constructing 

 it from what you have seen on several sections. 



b) Cross-section: Examine slide " Hydra trans." Make a careful study 

 with the high power of the cell structure. 



(1) Ectoderm: The ectoderm consists of a sheet of cells one cell thick, form- 

 ing a continuous outer layer of the animal, and appearing as the outer circle in 

 the cross-section. It is composed mainly of large epithelio-muscular cells whose 

 boundaries are often indistinct. The greater part of each of these cells is a 

 polyhedral epithelial cell, but the base is drawn out into a long, slender, muscular 

 fibril, running in a longitudinal direction. Thus the bases of all the ectoderm 

 cells produce a longitudinal muscle coat for the Hydra, by means of which it is 

 able to contract. (For the appearance of the muscle fibrils see below under 

 "Mesogloea.") Each epithelio-muscular cell has a large nucleus, easily recognized 

 by its black central nucleolus and distinct network. Within many of the 

 epithelio-muscular cells are numerous darkly stained nematocysts, of which the 

 various types described above will be recognized. Each is inclosed in its mother- 

 cell, the cnidoblast, which is a modified interstitial cell. These latter are small, 

 dark, granular cells, each about the size of a nucleus of the ordinary ectoderm 

 cells, and occurring in groups or masses which are within or between the epithelio- 

 muscular cells. 



(2) Entoderm: The entoderm like the ectoderm is a continuous sheet of 

 cells, forming a sort of inner tube for the animal. It is made up chiefly of the 

 nutritive muscular cells, or digestive cells, large, elongated, vacuolated cells, with 

 well-defined walls, and bulbous, crowded inner ends. They usually contain many 

 food particles and droplets, have a nucleus like that of the ectoderm cells and a 

 small amount of granular cytoplasm, confined mainly to the inner ends. Their 

 outer ends, next to the mesogloea, are also prolonged into muscular fibrils, which 

 run circularly, forming a circular coat, but this is relatively poorly developed as 

 compared with the outer longitudinal coat. Situated between the inner ends of the 

 digestive cells is another type of cell, the secretory or glandular cell, which produces 

 the digestive enzymes. This appears as a small triangular cell, filled with a 

 network, resting between the diverging inner ends of the adjacent nutritive cells. 



(3) Mesogloea: The mesogloea is not a cell layer but a sheet of gelatinous 

 material cementing the ectoderm and entoderm together. The longitudinally 

 directed muscular bases of the ectoderm cells are imbedded in the mesogloea and 

 appear there as dark dots. 



