SECTION VIII 



The Circulatory System 



The actual blood-vessels in Apus are limited to the 

 long dorsal vessel or heart. Although, among the 

 Annelida, circulatory systems are found of many 

 different grades of development, it is not necessary 

 to suppose that our original Crustacean-Annelid had 

 a very simple blood vascular system, in order to 

 account for the above fact. It seems to us probable 

 that the development of an exoskeleton, which holds 

 the organs in their places, and protects the inner 

 parts generally from being squeezed together, renders 

 special blood-vessels more or less unnecessary, the 

 blood being able to bathe all the organs of the body 

 without difficulty. On the other hand, this is not the 

 case in a soft-skinned strongly contractile body, such 

 for instance as that of the leech, where it is necessary 

 to carry special blood streams between the organs 

 which are liable to be crowded together. If then our 

 original Crustacean-Annelid possessed a highly deve- 

 loped blood vascular system, it would naturally, with 



