EVOLUTION OF THE VERTEBRATE 181 



It is true that in Aurelia they all end by becoming discon- 

 tinuous, but it requires no great effort of imagination to 

 picture a primitive type in which the successive medusiform 

 parts of the strobila did not separate, but were obliged to 

 develop in close continuity, so that a series of interdependent 

 segments was the result. Such development in close con- 

 tinuity would in fact produce a primitive piscine organism ; 

 and we propose to try to show how a mouth, gills, brain 

 and cord, and closed circulatory system, such as possessed 

 by the typical Fish, would be almost inevitable consequence. 

 It cannot be emphasised too strongly that the primitive 

 Fish must have originated suddenly, and that its mature 

 plan was the result of forces moulding and guiding on new 

 lines the development of a fertilised ovum originally serially- 



Fig. 54. — The development of Aurelia. a, Scyphistoma ; 

 b, further stage ; c, strobilation begins ; d, strobila ; e, free- 

 swimming Ephyra. (After Claus.) 



medusoid in its potentialities. Thus we are not to picture 

 the modification of a strobilar plan already realised, but 

 the moulding, on new lines, of developing tissue whose 

 original idea was to realise a strobilar plan. 



The umbrella or bell of a medusoid is not only a 

 locomotive but also a respiratory organ. In humbler forms 

 of aquatic life the digestive is also the respiratory system, 

 as in the Hydra zooid, where the body-cavity not only 

 receives food for digestion but also oxygenated water for 

 respiration ; but in a medusoid, equivalent to a number 

 of zooids united closely in lateral Continuity, there is neces- 

 sarily more specialisation. For the somatic cavity, while 

 a simple tube to begin with, expands to form a dilatation 



