ON THE DIPLOCHORDA. 



325 



Rather than work back to simpler organisms, I would 

 attempt to follow the indicated course of evolution from simple 

 to complex. 



In woodcuts 6, 7, and 8 I have depicted the leading characters 

 of Stage I, a pelagic, triploblastic, radially symmetrical ancestor 



6. 



7. 



8. 



6. Ventral view of Stage I. 7. Ventral view of ditto. 8. Lateral view of 

 ditto. N.B. — In this and succeeding woodcuts the general ectoderm is 

 unshaded, tlie endoderm shaded, nervous areas black, and mesoderm 

 dotted. Tiie ccelomic pouches are numbered. 



of the CcBlomata, or at least of those Coelomata to which 

 I wish to refer. AVith four cojlomic pouches it is quite easy 

 to imagine this organism as derived from a pelagic Ccelenterate 

 with four gastric pouches. The origin of the radial symmetry 

 must be assigned to the conditions of free pelagic (floating) 

 existence, and the presence of the apical nervous system 

 precludes a sedentary origin. The environment, in a hori- 

 zontal plane, of a free pelagic organism with only perpendicular 

 automatic locomotion should be as capable of impressing a 

 radial symmetry upon an organism as that of a sedentary 

 habit. 



We may note that there is an apical ganglion with four 

 radial nerves, meeting a nerve-ring round the mouth. 



Below the nerve-ganglion is a subneural sinus which com- 



