HYDBOIDS 181 



As the constriction rings produced in these tests must 

 be the result of the contraction of localized parts of the 

 circular muscle and as their formation is quite indepen- 

 dent of anesthetization, it follows that the longitudinal 

 and circular muscles must act upon very different prin- 

 ciples. The longitudinal muscle contracts much more 

 quickly than the circular muscle does and its activity, un- 

 like that of the circular muscle, is quite abolished by such 

 drugs as chloretone. These features are the character- 

 istics of a muscle under nervous control as contrasted 

 with one brought into action by direct stimulation, and 

 it is, therefore, probable that the longitudinal muscle of 

 the stalk is normally under nervous influence, and that 

 the circular muscle is stimulated directly .3 



Under such circumstances the interplay between the 

 longitudinal and circular muscles in the shortening and 

 elongation of the stalk as a result of stimulation is easily 

 pictured. Any normal stimulus applied to the stalk gives 

 rise to the impulses that spread through the ectodermic 

 nerve-net and call forth an immediate contraction of the 

 longitudinal muscle. As a result the stalk shortens 

 nearly one-half and the axial cells become crowded to- 

 gether, causing it to thicken proportionally. In conse- 

 quence of this thickening the circular muscle is stretched, 

 its tonus is probably increased, and it is gradually ex- 

 cited to action, so that on the relaxation of the longitu- 

 dinal muscle the circular muscle contracts and forces the 

 axial cells back to their original form, thus elongating the 

 stalk. In this way the two muscles are brought into har- 

 monious action though only one of them is under control. 



The longitudinal and circular muscles of the proboscis 

 are probably organized upon much the same plan as 

 those of the stalk, but the proboscis is too small an organ 



