EFFECTOR SYSTEMS OF ACTINIANS 481 



of that process by which out of an undifferentiated state the 

 highlj^ complex nervous organization of the higher animals 

 has arisen. In my opinion there is not the least doubt that some 

 of the nem'omuscular responses in Metridium are true reflexes, 

 though the majority of such operations are more usually ex- 

 hibitions of excessive tonicity or of rhythmic motion. 



SUMMARY 



1. The effector systems of Metridium are at least four in 

 number : the mucous, the nematocyst, the ciliary, and the muscu- 

 lar system. 



2. The mucous, the nematocj^st, and the ciUary systems are 

 independent effectors and are not under the control of a ner- 

 vous mechanism. 



3. The muscular system, consisting of thirteen muscles or 

 groups of muscles, shows a variety of conditions. Some mus- 

 cles, such as the longitudmal muscles of the acontia, are in- 

 dependent effectors and are not under nervous control. Others, 

 like the circular muscles of the column wall may act indepen- 

 dently or under the influence of nerves. Still others, such as 

 the longitudinal muscles of the mesenteries, act only in response 

 to impulses from a relatively complex nervous mechanism. 



4. Non-nervous muscular responses are carried out sluggishly 

 and require a minute or more for completion. Nervous muscular 

 responses are relatively rapid and may be accompUshed in a 

 second or so. 



5. Notwithstanding that the whole musculature exhibits a 

 high degree of tonicity, there are responses such as the expansion 

 of the oesophagus by the action of the transverse muscles of 

 the complete mesenteries which are of the nature of well individual- 

 ized reflexes. 



