THE CIRCULATION OF THE BLOOD. 953 
marked that in this creature there are contractile dilatations at 
the bases of the branchial arteries. 
Fia. 227.—Circulatory and excretory organs of the cuttle-fish (Sepiu officinalis), viewed from 
the dorsal side (after Hunter). Br, gills; C, ventricle; do and Ao’, anterior and poste- 
rior aorta; V. lateral vein; Vc’, anterior vena cava; Vc’’, posterior vena cava; N, renal 
appendages of the veins; Vir, advehent branchial vessels (branchial arteries); Kh, 
branchial heart; Ap, appendage of the same; At, At’, auricles receiving the revehent 
branchial vessels (branchi: 
al veins). 
In some Ascidians the heart is of a somewhat. crescentic 
form, and has the remarkable property of beating for a time in 
one direction, then stopping and reversing its rhythm. In a 
transparent specimen, under the microscope, this can be seen 
admirably. 
In the crab the heart lies within a pericardium, loosely at- 
tached, the main vessels being connected with the pericardium 
and not directly with the heart. The heart sucks its blood 
from the pericardial cavity through four valvular openings. 
In such a creature as the scorpion there is a chambered 
heart, with a division for each principal segment of the animal’s 
body (Fig. 308). 
While in mollusks, crustaceans, and other groups, the vas- 
cular system does not form a connected whole, the scorpion is 
exceptionally advanced in this respect, being provided with 
capillaries, or tubes closely representing them. Among most 
of the invertebrates the blood, after leaving the arteries, passes 
into rather wide, irregular spaces among the various tissues, 
from which it is taken up by the veins without the intervention 
of an intermediate set of vessels. 
The circulatory system of an insect or crustacean may be 
