ON THE ANCESTRAL FORM OF THE CHORDATA. 313 



completely before the formation of cartilaginous branchial bars ; 

 so that any skeletal structures in front of the mouth, which have 

 been interpreted by morphologists as branchial bars, can never 

 have acted in supporting the walls of branchial clefts. 



(3) The region which, in the Vertebrata, forms the oeso- 

 phagus and stomach, was, in the ancestors of the Chordata, 

 perforated by gill-clefts. This fact, which has been clearly 

 pointed out by Gegenbaur, is demonstrated by the arrangement 

 of the gill-clefts in Amphioxus, and by the distribution of the 

 vagus nerve in the Vertebrata 1 . On the other hand the 

 insertion of the liver, which was probably a very primitive organ, 

 appears to indicate with approximate certainty the posterior 

 limit of the branchial clefts. 



With these few preliminary observations we may pass to the 

 main subject of this section. A fundamental question which 

 presents itself on the threshold of our enquiries is the differen- 

 tiation of the head. 



In the Chaetopoda the head is formed of a praeoral lobe and 

 of the oral segment ; while in Arthropods a somewhat variable 

 number of segments are added behind to this primitive head, and 

 form with it what may be called a secondary compound head. 

 It is fairly clear that the section of the trunk, which, in 

 Amphioxus, is perforated by the visceral clefts, has become the 

 head in the Vertebrates proper, so that the latter forms are 

 provided with a secondary head like that of Arthropods. There 

 remain however difficult questions (i) as to the elements of 

 which this head is composed, and (2) as to the extent of its 

 differentiation in the ancestors of the Chordata. 



In Arthropods and Chaetopods there is a very distinct 

 element in the head known as the procephalic lobe in the case of 

 Arthropods, and the praeoral lobe in that of Chaetopods; and 

 this lobe is especially characterized by the fact that the supra- 

 cesophageal ganglia and optic organs are formed as differentia- 



1 The extension forwards in the vertebrata of an uninterrupted body-cavity into 

 the region previously occupied by visceral clefts presents no difficulty. In Amphioxus 

 the true body cavity extends forwards, more or less divided by the branchial clefts, for 

 the whole length of the branchial region, and in embryos of the lower Vertebrata there 

 is a section of the body cavity the so-called head-cavities between each pair of 

 pouches. On the disappearance of the pouches all these parts would naturally coalesce 

 into a continuous whole. 



