GENERAL CONSIDERATIONS. 575 



We therefore regard Amphioxu8 as a very primitive chordate form 

 very closely related to the hypothetical ancestor of the Craniata, but 

 somewhat more distantly related to the Tunicates. Every specula- 

 tion as to the origin of the Vertebrates and the Chordata must 

 necessarily take account of Amphioxus as the most primitive repre- 

 sentative and the starting-point of the whole series. Among all the 

 hypotheses * which have hitherto been advanced as to the origin 

 of the Chordate stock, that which derives it from the Annelida has 

 at present most adherents. As the founders and most eminent 

 upholders of this hypothesis we must name SEMPER (No. 46) and 

 DOHRN (Nos. 30 and 31), while, more recently, a whole series of 

 renowned zoologists have taken part in its further development. The 

 view that the Vertebrates are descended from Annelids rests chiefly 

 on the similarity in the segmentation of the body and in the produc- 

 tion of new segments at the posterior end of the body ; further, on 

 the agreement in the position of the more important organs, if we 

 assume that the hypothetical Annelid ancestor of the Vertebrata 

 underwent such a rotation round its longitudinal axis that the former 

 ventral side became the dorsal side, an assumption which lay at the 

 root of GEOFFROY ST. HILAIRE'S statement that " Insects are 

 Vertebrates running on their back." When such a rotation is 

 assumed, the ventral ganglionic chain of the Annelida corresponds 

 to the medullary tube of the Chordata, the ventral longitudinal 

 vessel of the Annelida becomes the aorta, while the dorsal vessel 

 corresponds to the sub-intestinal vessel. The Annelidan hypothesis 

 obtained its strongest support when SEMPER discovered a remarkable 



bottom and fall on one side, is due to the physical impossibility to rest in any 

 other position, and not to a pressing desire or instinct to assume this position. 

 The asymmetry of Amphioxus is of a totally different character to that of the 

 r/c/ironectidae. For a full account of these views, see WILLEY, Amphioxus 

 -and the Ancestry of the Vertebrates, Columbia Univ. Biol. Series, 1894. ED.] 

 * We have no intention of entering upon the much-disputed point of the 

 origin of the Chordata except in a passing way. A detailed investigation of 

 this difficult problem would require a study of vertebrate embryology which 

 -does not fall within the scope of the present work. We have already, in the 

 foregoing chapter (p. 523), stated that the Tunicates contribute little to the 

 solution of this question. They are to be considered as degenerate members 

 of the Chordate stock, of which Amphioxus is to be regarded as the most 

 primitive form. Of the many theories on the subject of the relationship of 

 the Chordata we have here alluded only to those two which appear best 

 founded on actual morphological facts, viz., the derivation of the Chordata 

 from Annelids and the assumption of relationship between the Chordata and 

 Balanoglossns. The hypothesis of the relationship to the Nemertines has 

 been briefly alluded to above (vol. i., p. 231). We do not consider it necessary 

 to refer to the relationship of the Vertebrata to the Arthropoda which has 

 recently been again assumed. 



