454 G. H. PARKER AND E. G. TITUS 



It includes not only receptors and effectors but probably also 

 intermediate elements in the form of a nerve net through which 

 conduction is accomplished and in which it would not be sur- 

 prising to find the beginnings of central functions. 



The four tj^es of organization thus briefly described are 

 probably not simply types of structiire which meet special 

 requirements in the sea-anemone's activities, but they have, 

 we believe, a certain phylogenetic significance. The first, 

 the independent effector, represents the initial step in the evolu- 

 tion of the neuromuscular mechanism as realized in the sponges 

 (Parker, '10). The second and third, the combined receptor 

 and effector, introduce the first nervous element into this series 

 whereby greater efficiency in the discharge of the motor mechan- 

 ism is attained. And the fourth marks the first appearance 

 of that intermediate structure which in its incipiency is merely 

 a connecting and transmitting organ from receptor to effector 

 but which in its final outcome is the central nervous apparatus 

 of the higher animals. Thus the diversity of neuromuscular 

 organization in Metridium includes non-nervous as well as 

 nervous muscular response (Parker, '12) and seems to us to 

 have a certain significance for the phylogeny of the nervous 

 system. 



V. SUMMARY 



1. The muscular system of Metridium consists of thirteen 

 fairly well defined muscles or classes of muscles. 



2. These muscles represent at least four types of organization: 

 first, independent effectors, as seen in the longitudinal muscles 

 of the acontia; secondly, simple receptor-effector systems, hke 

 the circular muscles of the column, which, though brought into 

 action by direct stimulation, are probably also under some nerv- 

 ous control; thirdly, more highly specialized receptor-effector 

 systems like the longitudinal muscles of the tentacles, which 

 respond only through nervous stimulation; and fourthly, com- 

 plex receptor-effector systems as shown in the sense cells of 

 the column wall in conjunction with the longitudinal muscles 

 of the mesenteries. 



