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 
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 
continuous closed system of blood¬ 
vessels. 
As a rule, in all animals hav¬ 
ing any circulation at all, the cur¬ 
rent always takes one direction. 
This is generally necessitated by 
valves. But a curious exception 
is presented by the Ascidians, 
whose 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 
arterial bulb; c, ventricle; d, au¬ 
ricle ; e, venous sinus; /, portal 
vein ; g, intestine; h, vena cava; 
i, branchial vessels; k, dorsal ar¬ 
tery, or aorta; l, kidneys; m, 
dorsal artery. 
