68 PHYSIOLOGY 



rays (Fig. 43), while in the earthworm two long tubes 

 connected by a series of encircling rings run lengthwise 

 in the body (Fig. 44). 

 In Higher Animals. — In higher animals the number 



Fig. 43.— Diagram of the circulatory system of the starfish. 



of tubes is greater and their arrangement is consequently 

 more complex. There is a general similarity in all ani- 

 mals that have four limbs attached to a trunk. Minor 

 details such as the branching of the vessels may vary, but 



Fig. 44.— Diagram of the circulatory system of the earthworm. 



even from a frog one can get a fairly good idea of their 

 arrangement in man. 



The most noticeable feature of the system is an en- 

 largement called the heart. In very low animals it is 

 a simple tube. In higher animals it begins as a tube but 



