106 COMPARATIVE ZOOLOGY. 
whole of the blood, therefore, does not make a complete 
circuit. 
A still higher form is seen in the Cuttle-fish, the high- 
Samet 
Fig. 70.—Circulating Apparatus in 
the Fish: a, branchial artery; b, 
arterial bulb; ec, ventricle; d, 
auricle; e, venous sinus; J, port- 
al vein; g, intestine; h, vena 
cava; 7, branchial vessels; k, 
dorsal artery, or aorta; l, kid- 
neys; ™, dorsal artery. 
est of Invertebrates. Instead of 
a double heart, we find a triple 
one, a ventricle to throw the 
blood over the system, and two 
auricles to force it into the gills. 
Hitherto, there has been a pro- 
pelling cavity on the “ arterial” 
side only; but now the veins 
which collect the blood from the 
system to send it back to the 
heart by the way of the gills, 
are furnished with two branchi- 
al hearts, which accelerate the 
circulation through those organs. 
Many of the arteries and veins 
are joined by capillaries, but not 
all; so that in no invertebrate 
anifmal is the blood returned to 
the heart by a continuous closed 
system of blood-vessels. 
As a rule, in all animals hav- 
ing any circulation at all, the 
current always takes one direc- 
tion. This is generally necessi- 
tated by valves. but a curious 
exception is presented by the 
Ascidians (a group of low Mol- 
lusks), whose tubular heart is 
valveless, and the contractions 
occur alternately at one end and 
then the other; so that the blood 
oscillates to and fro, and a given vessel is at one time a 
