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- 
ing back 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 
