184 THE EVOLUTION OF CONTINUITY 



medusoid types. So, taking Fig. 57, A, to represent the 

 plan of the medusoid canal system, a series of such systems 

 joined up together through the intensification of megazooidal 

 serial Continuity would give a diagrammatic canal-plan 

 such as is drawn in Fig. 57, B. 



In humbler forms of segmental Individual, for example, 

 in some Annelids (from which, however, the Fish has not 



Fig. 57. — a, diagram of circulation of a medusa ; b, the result 

 of linking up a series of forms like A (leaving out the manu- 

 brium) ; c, diagram of the circulation of Saenuris, an Annelid, 

 for comparison ; h, heart. 



evolved), circulatory arch repetition is very regular down 

 the length of the animal, as is shown in C above. 



But in a medusoid the canal system is continuous with 

 the digestive tube, and communicates through this with 

 the mouth of the organism ; whereas in the Fish, while the 

 respiratory circulation is in close touch with part of the 

 digestive tract, all the bloodvessels are completely shut off 

 from this. And here we come to the crux of the question. 

 The medusoid has one opening which acts both as mouth 

 and anus, whereas in the primitive Fish an additional 



