VASCULAR ORGANS. . 19 
SYSTEM, which carries blood and is primarily a correlative 
system between the motor and alimentary on the one hand, 
and the respiratory and excretory on the other. Hence the 
blood is primarily a respiratory and excretory fluid. 
In most higher animals this system has a central organ 
of propulsion, the earz, to ensure proper circulation. In 
some cases, the heart drives the blood over the system, when 
it is called systemic, whereas in the others it propels the 
-blood directly to the respiratory organs, when it is known as 
a respiratory heart. Occasionally we find that the heart 
alternates in its action and it is then called reversible. 
The blood-system arises as a system of sinuses or spaces 
between the organs, in which condition it remains in the 
lower types; in higher types definite walls are formed and 
produce vessels. In those animals which possess a heart or 
central circulatory organ, the vessels carrying blood away 
from the heart are called arteries, those bringing blood to 
the heart are vezns. 
(2) THE C@ELOMIC SYSTEM, which usually carries a coe- 
lomic fluid. This fluid is primarily zw¢rztive in function but 
this function is often usurped by the blood-vascular system. 
In the forms with a nutritive ccelom the fluid bathes the 
muscles, gonads and skeletal system, and even in those cases 
in which the nutritive function is largely transferred to the 
blood, as in vertebrates, the ccelomic fluid (lymph) still acts 
largely as a medium of exchange between the tissues and the 
blood. Ccelomic hearts are not common, as the circulation 
of the fluid is usually assured by the movements of the body, 
but “‘lymph-hearts” are observable in the frog. 
Nervous System.—In the lowest types, the protoplasm 
of the body is alike irritable and contractile; but in the 
-higher organisms, as seen above, the property of contractility 
becomes concentrated in a motor system, and that of power 
of transmitting impulses in the sense-organs, The latter are, 
from their nature, bound to be situated peripherally, whilst 
the position of the former is determined by the mechanical 
principles of the body. Hence the necessity for a special 
means of direct communication between the two systems. 
The system which fulfils this condition is called the Nervous 
System. It first appears as connecting strands or nerves 
