16 REFLEX ACTION 



structure. For a solution of this problem we are naturally 

 led to the study of the evolution of organic life. 



RHYTHMICAL ACTIVITY 



Organisms perform many actions repeatedly which appar- 

 ently go on by themselves independently of outer stimulation. 

 Such actions may be viewed as responses, but the stimuli 

 causing them are internal instead of external. The beating 

 of the heart and the regular movements of respiration are 

 familiar examples of such rythmical activity. 



In the horse-shoe crab Limulus the abdominal appendages 

 Which bear the gills execute a regular to and fro movement 

 which will occur as well if the nerve cord is cut between the 

 thorax and the abdomen. Even if the nerve cord is cut 

 between the several abdominal ganglia the appendages will 

 still beat regularly, although all coordination of their 

 movements is destroyed. Among medusae there is a regular 

 contraction of the disk or bell during swimming which 

 usually disappears if the marginal nerve ring is removed, 

 but Loeb has shown that if the jelly fish Gonionemus, after 

 being operated on in this manner, is placed in sea water 

 devoid of calcium salts the rhythmical pulsations will 

 reappear. 



Among the Protozoa rhythmical movements are shown in 

 the regular swaying of Stentor, and the periodical contrac- 

 tions of Vorticella; and Loxophyllum, as is described in a 

 later chapter, shows a rhythmical alternation of movements 

 which are performed in much the same way by very small 

 pieces into which the infusorian may be divided. 



BIBLIOGRAPHY 



BETHE, A. Das Centralnervensystem von Carcinus moenas. Arch, 

 f. mik. Anat., 50 and 51, '97. Allgemeine Anatomie und Physi- 

 ologie des Nervensystems. Leipzig, '03. 



