THE ANCESTRY OE THE OHORDATA. 89 



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 

 larvae of Amphioxus, or may have crept in mud as the larvse 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 

 fulcrum, 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 



