494 THE ORIGIN OF VERTEBRATES 
of the Tunicata or Enteropneusta. Neither the Tunicata nor 
Amphioxus 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 evidence 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. Itis 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, 
