The visceral gangl'on of Pecten, etc. 
23 
It is perhaps hardly probable that two organs situated near each other 
and very different in structure should have an identical function, and 
in Pecten the abdominal sense organ is not in a favourable position for 
meeting the inhalent current. 
It is possible that this organ is for the regulation of the exhalent 
and inhalent currents, so important for nutrition and respiration. Both 
reproductive cells and faecal matter are expelled by sudden closure of the 
shell valves and the consequent expulsion of the water from the palliai 
cavity, but this takes place in a direction different again, from that in 
the swimming movements, or in overturning, and again the velum plays 
an important part. The latter may cause the water to escape, therefore, 
at each side near the hinge line (swimming), ventrally (overturning), 
or posteriorly (reproductive products etc.). The control of the currents 
in the latter case was seen in a rather interesting manner. Two specimen» 
of Pecten maximus were observed emitting spermatozoa in June, and after 
some trials, photographs of the process were obtained. The animai rested 
in the normal position with valves open and the two vela just touching, 
with however, a slight trace of an opening about two inches from the 
posterior end of the hinge line. Suddenly the valves were moved stili 
further apart and then foUowed a rapid closure. The two vela came 
together ventrally but an opening was left or rather made, near the poste- 
rior end, and a stream of spermatozoa shot out, like a jet of smoke from 
a gun (fig. 9), which diffused rapidly through the water until the whole 
tank was clouded. This emission of spermatozoa was not continuous 
and the process described above was repeated at intervals of 21/2 ™" 
nutes, 3 minutes, ^/^minutes etc. The action of the velum was under 
perfect control and also the currents in the palliai cavity, for after an 
emission of the spermatozoa I waited some time and then before the 
next expulsion could take place (it is always preceded by warning move- 
ments) the animai was disturbed. The result was a sudden closure of the 
valves, but no spermatozoa were emitted, although if the disturbane© 
was not continued the normal sequence of events immediately followed. 
The sperms appeared to be continuously poured out of the reproductive 
duct into the palliai cavity but not permitted to escape from the latter. 
I am afraid these observations are treated in a rather fragmentary 
manner here, but they show how intimately the velum, and the ad- 
ductor muscle work together and how the currents in the palliai cavity 
are regulated. 
One or two other results of Stimulation experiments remain to be 
referred to. Stimulation of the visceral ganglion, after Separation of this 
