PERIODS OF THE PULSATIONS. 247 



former exit, and passed out at their former entrance, 

 and performed the circulation in every respect the 

 same as before, but in the reverse direction. 



Those globules that pass through the vessel into the 

 foot-stalk appear to accumulate there as in a reser- 

 voir, until the course is changed ; when they crowd 

 into the heart again and perform their grand tour. 

 Yet there is a measure of circulation here, for even 

 in the connecting vessel one stream ascends from the 

 reservoir into the body as the other (and principal 

 one) descends into it from the heart ; and so, vice 

 versa. 



I have spoken of these motions as being performed 

 with regularity ; but this term must be understood 

 with some qualification. The pulsations are not quite 

 uniform, being sometimes more languid, sometimes 

 more vigorous ; perhaps forty beats in a minute may 

 be the average ; but I have counted sixty, and pre- 

 sently after thirty ; I have counted twenty beats in 

 one half-minute, and only fifteen in the next. The 

 period during which one course continues is equally 

 uncertain ; but about two minutes may be the usual 

 time. Sometimes the pulsation intermits for a second 

 or so, and then goes on in the same direction ; and 

 sometimes there is a curious variation in the heart's 

 action, — a faint and then a strong beat, a faint and a 

 strong one, and so alternately for some time. 



Several other points in the organization of this 

 animal I might notice ; as the forked muscular bands 

 that ramify from each aperture, the use of which is 

 doubtless to perform the strong retractations of those 

 orifices ; and the curious knobbed or hooked processes 



