INDEPENDENT EFFECTOBS 63 



but extending several centimeters away from it, are vari- 

 ously stimulated at their free ends, not the least response 

 has ever been observed in the sea-anemone itself, though 

 the acontia react vigorously in the region to which the 

 stimulus is applied. The stimulation of their free ends 

 seems to have no more influence on the sea-anemone than 

 the cutting of the free end of a long hair has on a human 

 being. From these observations it seems fair to conclude 

 that the acontia of sea-anemones are devoid of nervous 

 structures and that their longitudinal muscle must, there- 

 fore, be stimulated directly as an independent factor 

 (Parker and Titus, 1916). 



From the instances thus far given it is evident that 

 independent effectors in the form of muscles occur among 

 the most differentiated as well as among the simplest of 

 the multicellular animals. The capacity of these effectors 

 to be stimulated directly is only another aspect of what 

 physiologists have long recognized in respect to ordinary 

 muscle, namely, the great ease with which such tissue can 

 be directly stimulated by almost any agent. The number 

 of these independent effectors will doubtless increase as 

 the animal series is more fully investigated. Some of 

 those already noted, like the acontial muscles of sea-anem- 

 ones, may be survivals of that primitive state seen in 

 the sponges; others, like the muscle of the embryonic 

 heart in vertebrates, may be special adaptations newly 

 brought into being by the exigencies of the particular sit- 

 uation. But however this may be, these examples all 

 point to the principle that of the three elemental constitu- 

 ents of the neuromuscular mechanism, the sense organ, 

 the central nervous organ, and the muscle, the only one 

 that can be thought of as existing independently is the 

 muscle, and that this, therefore, is the most primitive 

 of the three. 



