76 LABORATORY STUDIES IN ZOOLOGY 



iiulivicliuil. Altliough hcrnuiphroditic it is rather excepticjiial 

 to find specimens bearing both ovaries and spermaries at the 

 same time. 



Make an outline drawing, at least five inches in length, of an 

 expanded Hydra as seen from the side. To the same scale 

 draw the animal in a contracted state. 



Details of Cellular Structure. — Examine a stained cross-section 

 of the animal using the low power for the preliminary study. 

 Hydra clearly presents the fundamental plan of structure in 

 the coelenterates. Observe that the cells of the body wall are 

 arranged in two definite laj^ers, an outer ectoderm and an inner 

 entoderm, the latter forming the lining of the gastrovascular 

 cavity. These two layers are sharply marked off from each other 

 by a very thin non-cellular layer of supporting substance, the 

 mesoglea. 



Study the cross-sections under the high power. The ectoderm 

 is composed of three principal kinds of cells. By far the common- 

 est type is the epithelio-museular cell to be distinguished by its 

 large nucleus and deeply staining nucleolus. The bases of these 

 are expanded and contain long contractile fibers which form a 

 thin longitudinal muscular layer accounting for the remarkable 

 capacity for contraction which Hydra possesses. These con- 

 tractile bases cannot be made out in thin cross-sections. 



A second type of ectodermal cell is called the interstitial cell. 

 These are small, spherical, somewhat granular cells occurring in 

 groups between the epithelio-muscular cells. They appear to 

 be undifferentiated cells held in reserve to take up any one of 

 several functions when necessary. It may be that they give 

 rise to cnidoblasts more frequently than to other kinds of cells. 



The cnidoblasts are the cells enclosing capsules or nematocysts 

 containing the stinging threads. Each cnidoblast contains a 

 nucleus and gives rise to a spinelike projection, the cnidocil, 

 at its outer end. Nematocysts show an affinity for eosin and 

 therefore appear stained a pinkish color when treated with this 

 dye. Threads shot out from these cells penetrate the body of 

 small animals and inject into them a fluid which has a paralyzing 

 effect. 



At least two types of cells can be distinguished in the entoderm. 

 The more common type is the large, elongate, vacuolated diges- 

 tive cells. They are dumb-bell-like in shape, the larger end 

 extending into the gastrovascular cavity. Their nuclei and 



