ORGANS OF RESPONSE 173 



sion or a combination of both. Very often, however, response 

 takes the form of glandular activity. Some times light is 

 produced and some times an electrical discharge. The latter 

 responses are relatively rare. We will now 

 consider the organs of response; and first the 

 muscles. 



407. When an expanded amoeba is strongly 

 stimulated it contracts into a spherical mass. 

 How this is done we do not know. It is a 

 property of undifferentiated protoplasm in 

 which no contractile elements of any kind can 

 be distinguished. In some other protozoa 

 (paramcecium and stentor) there are distinct 

 contractile elements in the form of slender 

 fibrils (myonemes), which traverse the ecto- 

 plasm in a longitudinal, slightly spiral direc- 

 tion. By their contraction they also cause the 

 animal to assume a more nearly spherical form. 

 Such cells contract more energetically than 

 does the amoeba. 



408. In hydra, the contractile fibrils of each 

 cell are grouped into a bundle, the muscle 

 fibre, which is much longer than the body of 

 the cell and projects on either side. This gives 

 the cell the form of a T with a very short stem, 

 representing the cell-body and the cross bar 

 representing the fibre. The fibres of the ecto- 

 derm cells run longitudinally, while those of 

 the entoderm run circularly around the body. 

 In this case part of the cell remains undiffer- 

 entiated and continues to form part of an 

 epithelium. This condition is also met with in others of the 

 lower phyla, but in the Annelids and higher forms the differentia- 

 tion proceeds farther and involves the entire cell. So we find 



