118 THE EVOLUTION OF CONTINUITY 



and its movements be based on ancestral medusoid bell- 

 contractions. 



With the help of some diagrams we shall make clearer 

 the broad outlines of our hypothesis, which will be developed 

 further when we come to the evolution of the primitive 

 Fish. 



Let us take a segment of the earthworm between the 

 seventh and the twelfth ; that is, one which contains a heart. 

 In the muscularly contractile walls of the segment we would 

 recognise the contractile ancestral medusoid bell. Sur- 

 rounded by these walls is the ccelomic space, and this we 

 take to represent the space between bell and manubrium 

 in the ancestral medusoid, while the transverse septa dividing 

 off the segments might represent medusoid vela developed 

 olosely round the alimentary tube. Down the centre of the 

 segment runs the alimentary canal closely encircled by a 

 heart of the worm. In the alimentary tract we recognise 

 the ancestral medusoid manubrium, and in the heart a 

 medusoid water-vascular circular canal. 



In the mysterious Echinoderm, in the Sea-urchin for 

 example, one comes across a parallel exhibition of the close 

 encirclement of the digestive tube by a circular vessel ; 

 and although here the vessel is not a true water-vascular 

 one of medusoid type, nor yet a true bloodvessel, we believe 

 that it represents an ancestral medusoid circular canal. For 

 it encircles the oesophagus, and from it radiate five canals 

 which bear the protrusible tube-feet of the organism. The 

 radial canals do not communicate with the alimentary tract 

 as in the medusoid, and their function is in connection with 

 locomotion ; and it seems most probable that this state 

 of affairs came about with, or as a result of, the development 

 of an anus in the first primitive echinoderm. And in the 

 sea-urchin this small circular canal is accompanied by a 

 closed ccelom which we would suppose to be originally due 

 to the closing in of the medusoid velum. 



But in the primitive annelid we believe the mouth to have 

 been a new feature ; that is, that what we picture as the 

 mouth end of a strobilar medusoid series took on the 

 functions of an anus — having ancestrally acted both as 

 entrance and exit — and that an opening developed at the 

 convex end of the series and became the mouth ; all this 



