208 MOLLUSCA. 



Another larger portion of the blood, on leaving the heart, 

 immediately divided into many ramifications that spread like 

 a network over the stomach and intestines and the soft sub- 

 stance of the mantle. Of these a part run into the horizon- 

 tal passages above the branchial sac, a part into the descend- 

 ingback stream ; a large portion, after leaving the intestines, 

 took a short course, and, collecting into one channel, flowed 

 into that stream near the bottom, and, all united, then 

 entered the peduncle and constituted the returning current 

 that went to circulate in other animals of the group. 



After this circulation had gone on for a while the pulsa-. 

 tions became fainter for a few beats, and the flow slower, and 

 suddenly, with but a slight pause, the whole current in all 

 its windings was reversed. .The heart gave the opposite 

 impulse ; the channel in the peduncle, that before poured 

 in the blood, now carried it back, and the other the con- 

 trary, and every artery became a vein. These changes 

 continued and succeed each other alternately, the average 

 time of the currents being the same in both directions, but 

 the period of each varying within a single observation as 

 much as from thirty seconds to two minutes. The pheno- 

 menon, like the currents in the Sertulariae, was invariably 

 met with in every animal of the species that came under my 

 notice. 



Sometimes, when the creeping tube or the peduncle has 

 been injured, the circulation of an individual is in conse- 

 quence insulated, but without appearing to impair any of its 

 functions. I severed one at the part where it joined the pe- 

 duncle, when for a few seconds the pulsation ceased ; it then 

 began irregularly and with considerable pauses, and increased 

 in steadiness as it went on. At first the impulse given by the 



