EVOLUTION OF THE VERTEBRATE 195 



alimentary tract does not lend real support to Gaskell's 

 theory, in our opinion. In fact, we believe that the phe- 

 nomena of early development are not safe guides to the 

 derivation of mature structure. Thus, for example, in the 

 same case of Amphioxus, the ccelom is usually described 

 as arising from the developing alimentary tract in the form 

 of two outgrowths or protrusions. Our belief, as already 

 clearly stated, is that in no living organism can the ccelom 

 be regarded as having been derived in the evolutionary 

 sense from alimentary structure ; and the most we are 

 entitled to say is that in the developing Amphioxus there 

 is at first a common cavity from which is formed both 

 ccelom and alimentary tract. It is not suggested that the 

 study of early development is not of the greatest value in 

 the case of different species within a given type of Continuity, 

 but our suggestion is that where the question is the evolu- 

 tion of a given type of Continuity from a lower one the 

 comparison of mature structure is the safer guide. 



As regards the Vertebrate alimentary tract, this we 

 believe represents ancestral medusoid manubria united in 

 serial continuity ; in harmony with the explanations already 

 given of the evolution of the vascular and nervous systems. 

 The abdominal cavity, or coelomic space, in which the 

 digestive tract is loosely suspended, would thus be equivalent 

 to a number of medusoid " open cceloms " merged in serial 

 continuity (see page 186) ; the " open ccelom " being the 

 space obtaining between the medusoid's manubrium and 

 inner bell-surface. The significance of the true ccelom has 

 already been mentioned in connection with the earthworm 

 and the echinoderm. 



