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 

 Mood is primarily a respiratory and excretory fluid. 



In most higher animals this system has a central organ 

 of propulsion, the heart, 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 veins. 



(2) The ccelomic system, which usually carries a coe- 

 lomic fluid. This fluid is primarily nutritive in function but 

 this function is often usurped by the blood-vascular system. 



In the forms with a nutritive coelom 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 



