80 THE EVOLUTION OF MAN. 



nervous system in the form of a medullary tube ; below, on 

 the ventral side, is the intestinal tube, the anterior half of 

 which is a respiratory gill-intestine, its posterior half a 

 digestive stomach-intestine. It is true that the free- 

 swimming larva of the extant Ascidian possesses this typical 

 vertebrate character only for a short time ; it soon relin- 

 quishes its free roving mode of life, puts off its oar-like tail 

 with the notochord, adheres to the bottom of the sea, and 

 then undergoes that very great retrogression, the surprising 

 final result of which we have already observed (Chapters 

 XIII. and XIV.). Nevertheless, the Ascidian larva, in its 

 very transitory evolution (for a brief space), affords us a 

 picture of the long extinct Chordona-form, which must 

 be regarded as the common parent-form of Mantle-animals 

 and Vertebrates. There is even yet extant a small and 

 insignificant form of Mantle-animal which throughout life 

 retains the structure of the Ascidian larva with its oar- 

 like tail and its free-swimming mode of life, and which 

 reproduces itself in this form. This is the remark- 

 able Appendicularia (Fig, 187), which we have already 

 examined. 



If we ask ourselves what conditions of adaptation could 

 possibly have had so remarkable a result as the develop- 

 ment of the notochord, and the modification of a branch 

 of the Soft-worms into the parent-form of the Chorda- 

 animals, we may with great probability answer, that this 

 result was effected by the habituation of the creeping 

 Soft-worm to a swimming mode of life. By energetic and 

 continued swimming movements, the muscles of the trunk 

 would be greatly developed, and a strong internal point of 

 attachment would be very favourable to this muscular 



