180 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



active than the brown ones, frequently change their position if food 

 is not abundant. They respond to chemical stimuli of food, to light, 

 and to unfavorable temperatures, food being the chief factor in their 

 environment. The color of green hydras is due to the presence of 



Hydra is able to change its position both by turning "handsprings" as shown 

 in the diagram and also by contraction and expansion of the basal portion of the 

 body. 



minute green algae, called Zoochlorellae, that live in a symbiotic 

 relationship within the endodermal cells. 



The term, Coelenterata, which is the name of the phylum to which 

 the common Hydra vulgaris belongs, comes from the Greek words 

 koilos, hollow, and enteron, intestine, which may be translated "hav- 

 ing an internal digestive cavity," an apt title, since a Hydra is really 

 a hollow, double-walled bag. 



The Ectoderm and Its Functions 



The bulk of the outer layer of cells (ectoderm) is made up of large 

 epitheUo-muscular cells, having a layer of muscle fibers placed lon- 

 gitudinally at their bases, that enable the animal to lengthen or 

 shorten its body. A similar layer of fibers on the inner layer of cells 

 which run circularly around the body allows it to expand or contract 

 in diameter. Between the epithelio-muscular cells and near the inner 

 margin of the ectoderm are found numerous smaller interstitial cells 

 from which are derived numerous other cells, including the cnido- 

 blasts. Nerve cells are likewise scattered throughout the ectoderm, 

 forming a nerve Jiet at the base of the epithelial cells. 



Cnidoblasts are most abundant on the tentacles, although they are 

 found on all parts of the body exclusive of the basal disk. They hold 

 four kinds of stinging capsules, nematocysts, by means of which the 

 animal paralyzes living prey that comes in contact with its tentacles. 

 The nematocysts are capsules containing a hollow inverted thread 

 which under certain conditions can be thrown out, together with a 

 poisonous substance, hypnotoxin, that has the power to paralyze any 

 other small animal which it touches. The nematocyst reacts to cer- 



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