60 MANUAL OF ANIMAL BIOLOGY 



Ectoderm. As will be noted in the succeeding paragraphs, there 

 are several kinds of ectoderm cells. However, by far the greatest 

 number of the ectoderm cells are known as the epithelio-muscular 

 cells which form the covering, or epithelium, of the body. As 

 indicated by their name, these epithelial cells also contain contrac- 

 tile elements located near their inner ends, which function in a 

 similar way to the muscle cells in the higher types of animals. 

 In the basal disc, some of the epithelio-muscular cells also secrete 

 a sticky fluid which enables the Hydra to attach itself to various 

 objects. 



One end of the epithelio-muscular cells is typically somewhat 

 larger than the other, and these large ends, closely fitted together, 

 form the outer surface of the body. The inner portions of these 

 cells are generally not in close contact with each other, and 

 the spaces between often contain another type of ectodermal cell 

 known as the interstitial cells. Some of these interstitial cells 

 may become further differentiated into either cnidorlasts or into 

 specialized germ cells. The latter will be considered in the sec- 

 tion on reproduction. 



The cnidoblasts, developed from the interstitial cells, become 

 highly specialized to form the defensive and offensive weapons of 

 Hydra, known as nematocysts, or stinging capsules. Except in 

 the region of the basal disc, they are scattered in considerable num- 

 bers over the entire surface of the body. They are present in the 

 greatest numbers on the tentacles, where they are consolidated to 

 form ' batteries of nematocysts' which may be easily seen as bands 

 encircling the tentacles. Occupying the center of this type of cell 

 is a remarkable structure, the nematocyst proper, which consists 

 essentially of a long, coiled, filamentous tube attached to a spine- 

 covered base. The tube contains a poisonous substance. A rather 

 rigid spine, the cnidocil, projects from the outer surface of these 

 cells into the water surrounding the animal. It is apparently 

 sensitive to contact. 



Under the proper conditions, the nematocyst can be exploded. 

 When this occurs, the coiled, poisoned tube bursts through the 

 surface of the cnidoblast, uncoils with great rapidity and with suffi- 

 cient power to penetrate the tissues of various small animals with 

 which Hydra comes into contact. In such a case the attacked 

 animal, paralyzed by the action of the poison, is captured by the 

 tentacles of Hydra and conveyed to the mouth. In addition to the 



