494 THE ORIGIN OF VERTEBRATES 



of the Tunicata or Enteropneusta. Neither the Tunicata nor 

 Ainphioxus can by any possibility be on the direct line of ascent 

 from the invertebrate to the vertebrate. They must both be looked 

 upon as persistent failures, relics of the time when the great change 

 to the vertebrate took place. The Enteropneusta are on a different 

 footing; in their case any evideuce of affinity with vertebrates is 

 very much more doubtful. 



The observer Spengel, who has made the most exhaustive study 

 of these strange forms, rejects in toto any connection with vertebrates, 

 and considers them rather as aberrant annelids. The so-called 

 evidence of the tubular central nervous system is worth nothing. 

 There is not the slightest sign of any tubular nervous system in the 

 least resembling that of the vertebrate. It is simply that in one place 

 of the collar-region the piece of skin containing the dorsal nerve of 

 the animal, owing to the formation of the collar, is folded, and thus 

 forms just at this region a short tube. My theory explains in a 

 natural manner every portion of the elaborate and complicated tube 

 of the vertebrate central nervous system. In the Balanoglossus 

 theory the evolution of the vertebrate tube in all its details from this 

 collar-fold is simple guesswork, without any reasonable standpoint. 

 Similarly, the small closed diverticulum of the gut in Balanoglossus, 

 which is dignified with the name of " notochord," has no right to the 

 name. As I have already said, it may help to understand why the 

 notochord has such a peculiar structure, but it gives no help to 

 understanding the peculiar position of the notochord. The only 

 really striking resemblance is between the gill-slits of Amphioxus 

 and of the Enteropneusta. In this comparison there is a very great 

 difficulty, very similar to that of the original attempts to derive 

 vertebrates from annelids — the gill-slits open ventrally in the one 

 animal and dorsally in the other. In both animals an atrial cavity 

 exists which is formed by pleural folds, and in these pleural folds 

 the gonads are situated so that the similarity of the two branchial 

 chambers seems at first sight very complete. In the Enteropneusta, 

 however, there are certain forms — Ptychodera — in which these pleural 

 folds have not met in the mid-line in this branchial region, and in 

 these it is plainly visible that these folds, with their gonads, spring 

 from the ventral mid-line and arch over the dorsal region of the 

 body. Equally clearly Amphioxus shows that its pleural folds, 

 with the gonads, spring from the dorsal side of the animal, 



