Or 
Or 
~I 
THE ANCESTRY OF THE CHORDATA. 
The Habits of Life and Form of the Body of the Primitive 
Chordata. 
Habits of Life.—The presence of gill-slits in all the 
Chordata may be taken as positive evidence that they arose in 
an aquatic habitat. Moreover, such a structure as the noto- 
chord cannot be conceived as having arisen in a fixed form. 
Hence they probably led a more or less free existence. This 
being so, they may either have been pelagic creatures, as the 
larvee of Amphioxus, or may have crept in mud as the larvee of 
B. Kowalevskii. Between these two possibilities there is 
little or no determining evidence. The only feature which 
seems likely to affect the question is the question as to the 
original point in the body at which the notochord first segre- 
gated itself from the gut. Unfortunately the evidence upon 
this point is divided. For if we suppose that the condition in 
Balanoglossus is primitive, and that notochord began as a rod 
in the dorsal wall of the anterior end of the hypoblast, then 
this origin would more or less point to a burrowing habit, the 
notochord functioning as a support for the head in this opera- 
tion; but if the separation of the notochord in the middle of 
the body, as in Amphioxus, be held to be primitive, then this 
would point to a pelagic habit, the notochord serving as a 
fulerum, from which the movements of the animal in swimming 
might be maintained. The absence of fins on the young Bala- 
noglossus and on the young Amphioxus, though pelagic, 
appears to point slightly in favour of a burrowing habit, though 
no reliance can be placed on such slight negative features. 
Primitive Mouth.—There is one more point that does 
point in favour of a pelagic habit, namely, the fact that the 
anteriorly-directed digging mouth of both Balanoglossus and 
of Amphioxus is of secondary origin, being formed by a modi- 
fication of a more primitive ventrally-directed mouth. 
Balfour, having the mouth of Lampreys and Tadpoles in 
view, held that the original Vertebrate mouth was suctorial. 
This the ventrally-directed mouth might have been; but this 
VOL, XXVI, PART 4,—NEW SER, 00 
