182 THE EVOLUTION OF CONTINUITY 



from which radiate the nectocalycine canals to join the 

 circular canal in the rim of the bell. In this way the diges- 

 tive canal is continued as a water-vascular system. The 

 canals, in fact, act as primitive bloodvessels in that they 

 convey to all parts of the organism oxygen for respiration, 

 in addition to nutritive substances from the digestive cavity 

 proper. 



In all probability bell contractions and relaxations are 

 the chief agencies in maintaining a proper canal circulation, 

 the former causing the expulsion of fluid bearing impurities, 

 and the latter causing an inflow of fresh water bearing 

 oxygen and food. The regularly recurring accumulation of 

 waste-products no doubt stimulate the nerve-ganglia in 

 the bell-rim to call for bell-contraction, and thus it is that 

 in the free-swimming medusoid there appears for the first 



Fie. 55. — a, diagram of the gills on the left side of a typical 

 Fish, b, diagram of a pair of laminae, a, the artery which 

 gives off little branches which are linked up through capillaries 

 with the little venules leading to the vein, 6. (After Nicholson.) 



time in the ascending scale of types rhythmic respiration 

 due to muscular action. 



In the typical Fish this bell-contraction is represented 

 by gill-contraction, and when we examine the vascular 

 anatomy of the Fish's respiratory system we get some light 

 on the mode of evolution followed. It is not to be supposed 

 that the head of the Fish represents an ancestral medusoid ; 

 in reality it represents an ancestral series terminally com- 

 pressed so that the segments are indistinguishable ; the 

 result of development while moving forward in the face of 

 water-pressure, and reflected in the formation of the brain- 

 mass, as will be shown. 



It is common knowledge that the Fish breathes by 



