THE CIRCULATION OF THE BLOOD. 
107 
A still higher form is seen in the Cuttle-fish, the high¬ 
est of the Invertebrates. This animal has a central heart, 
with a ventricle and two auricles, 
and, in addition, the veins which a \ 
collect the blood from the system 
to send it back to the heart by 
the way of the gills are furnished 
with two branchial hearts , which 
accelerate the circulation through 
those organs. Many of the arte¬ 
ries and veins are joined by cap¬ 
illaries, but not all; so that in 
no invertebrate animal is the 
blood returned to the heart by a 
continuousclosed system of blood¬ 
vessels. 
As a rule, in all animals hav¬ 
ing any circulation at all, the cur¬ 
rent always takes one direction. h 
This is generally necessitated by 
valves. But a curious exception 
is presented by the Ascidians, 
w T hose tubular heart is valveless, 
and the contractions occur alter¬ 
nately at one end and then the 
other; so that the blood oscil¬ 
lates to and fro, and a given ves¬ 
sel is at one time a vein and at 
another an artery. In this re¬ 
spect it resembles the foetal heart 
of higher animals (Fig. 279). 
In Vertebrates only is the cir¬ 
culating current strictly confined 
to the blood-vessels; in no case does it escape into the 
general cavity of the body. In other respects, there is 
Fig. 71.—Circulating Apparatus in 
the Fish: a, branchial artery; b, 
arterial bulb; c , ventricle ; d , au¬ 
ricle ; e, venous sinus; /, portal 
vein; g, intestine; h , vena cava; 
branchial vessels; k , dorsal ar¬ 
tery, or aorta; l, kidneys; m, 
dorsal artery. 
