xvii PBOTOCHOEDATA 605 



THE AFFINITIES OF AMPHIOXUS WITH THE ENTEEOPNEUSTA 



We have now to endeavour to answer two questions, viz. (a) Does 

 the developmental history of Am^hioxus, as we have sketched it, 

 afford support to the idea that the Euteropneusta have vertebrate 

 affinities; and (ft) How are we to explain the extraordinary asym- 

 metrical larva, which nevertheless becomes metamorphosed into an 

 adult which is nearly bilaterally symmetrical ? 



With regard to the first question, it must be remembered that 

 even if Enteropneusta be admitted to the Vertebrate phylum, they 

 must, nevertheless, be widely separated from Cephalochorda. Between 

 a grade of structure in which the neural tube was confined to the 

 collar region, and in which the notochord is represented by a small 

 anterior diverticulum of the pharynx destined to support the base of 

 the proboscis region ; and a grade of structure in which neural tube 

 and notochord extend throughout the entire length of the body, an 

 enormous gap intervenes, to span which an enormous lapse of time 

 must have been required. 



Nevertheless, in the segmentation of the egg, and in the forma- 

 tion of the layers, there is a fundamental agreement between 

 Enteropneusta and Cephalochorda. The segmentation of the egg of 

 Dolichoglossus, as sketched by Davis, appears to be of the same general 

 type as that of the egg of Amphioxus. The same hemispherical 

 blastula stage is found in Amphioxus and in Balanoglossus, as de- 

 scribed by Heider ; and in both cases the archenteron is large and 

 nearly fills the blastocoele, and the embryo, immediately after 

 gastrulation, grows markedly in length. In both cases too (if we 

 follow Morgan [1891] in his account of the development of the New 

 England Tornaria larva), the mesoderin arises from the archenteric 

 wall as five outgrowths, viz. an anterior unpaired pouch and two pairs 

 of lateral pouches. In both the neural plate arises in the same way, as 

 do also the protecting flaps of ectoderm which cover it in. In both 

 it is eventually converted into a neural tube. In both the left division 

 of the anterior coelomic pouch opens to the exterior by a pore. When 

 we add to these resemblances the anatomical similarities between 

 the adult forms in respect of notochord, gill-slits, and tongue-bars, it 

 will be admitted that a very strong case for the vertebrate affinities 

 of the Enteropneusta has been made out. We have not to reconstruct 

 a single organ, and we take as our starting-point a very simple free- 

 swimming animal represented by the Tornaria larva. 



On any other theory of the origin of Vertebrata, such as the idea 

 that they carne from Annelida, a new mouth has to be manufactured 

 and other radical changes in function have to be postulated, for 

 which there is no precedent in the evolutionary changes which we 

 can determine with moderate certainty. 



From the conclusion that the Euteropueusta are Vertebrata, some 

 very interesting consequences follow. The proboscis-cavity of the 

 Enteropneusta is represented by the " head-cavities " of Amphioxus, 



