AN ICHTHYOBDELLID PARASITIC ON SAND WHITING. 31 
like a fir cone, measuring 80 /u long, and are made up of 
about eight cells. In some cases these cells become separated 
and present the appearance in section shown in PI. 2, fig. 11. 
The valve placed just before the intestinal sinus is twice the 
length of the others. 
Circulation of the Blood. 
I have investigated the circulation of the blood and the 
lymph in living specimens found on whiting which I caught 
on the ocean beach. The chief mechanism for propelling the 
blood is the peristaltic contraction of the muscular wall of 
the intestinal sinus, first described by Johansson (1896). 
This peristalsis occurs in the three pairs of pouches of the 
thick-walled intestine and also in the rudimentary pouch. In 
the living animal the whole of the thick-walled intestine is in 
a state of active contractile movements. These begin where 
the thick-walled intestine passes into the rectum. The general 
movement is from behind forwards. The whole of the con- 
tractile muscular wall appears to contract simultaneously 
when the animal is very active, but in specimens in which the 
rate of contraction is lowered it is seen that the peristalsis is 
from behind forward. There is seen, however, a certain 
amount of individual contraction of separate paired caeca. 
The most active specimens I examined showed a rate of con- 
traction of over thirty times per minute. The blood forced 
forward by the contractions is prevented from flowing back 
by the valves of the dorsal vessel. 
In the dorsal vessel the backward pressure of the blood 
causes the valve to press against the sphincter fibres of the 
dorsal vessel just behind it. Very rapidly the blood passes 
onward, and the next valve acting in the same manner, the 
first valve is again forced forward by the incoming blood. 
The constriction of the dorsal vessel is much greater at the 
sphincter muscle fibres than elsewhere. So great indeed is 
the contraction here that the dorsal vessel resembles a string 
of sausages. The constrictions being at the points occupied 
