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 codrdination of their 
movements is destroyed. Among meduse 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 
Betas, A. Das Centralnervensystem von Carcinus mcenas. Arch. 
f. mik. Anat., 50 and 51,’97. Allgemeine Anatomie und Physi- 
ologie des Nervensystems. Leipzig, ’03. 
