INTERNAL ANATOMY. 75 



process of cephalisation, by which the Vertebrate head has 

 been evolved. They are innervated exclusively by the 

 cranial nerves, and in fact are considered as forming part 

 of the head. In Amphioxus there is, broadly speaking, no 

 head, and the region of the gill-slits forms part of the trunk. 

 In the evolution of the Craniota, therefore, what has hap- 

 pened is that the gill-clefts have been relegated to the 

 head, while the excretory tubules have become confined to 

 the trunk, and have ceased to occur in the neighbourhood 

 of the gill-clefts. Only the anterior region of the pharynx 

 of Amphioxus is represented by the pharynx of the higher 

 forms. The greater part of it corresponds to the unper- 

 forated portion of the alimentary canal, which follows 

 immediately behind the pharynx in these forms, extending 

 to the liver. 



We have referred above to the absence of a pronephric 

 duct in Amphioxus. Although this is true in the strict 

 sense of the term, yet Boveri gives reasons for supposing 

 that the right and left pronephric ducts are in a measure 

 represented by the right and left halves of the atrial 

 chamber. (Cf. Fig. 35, A and B). We will first glance 

 briefly at the mode of 



Development of the Atrial Cavity. 



For the sake of avoiding complications, it will be well to 

 confine the description at present to the mode of origin of 

 the atrial cavity in its posterior region. It arises of course 

 on the same principle throughout its whole extent (except 

 the post-atrioporal continuation, which ^ grows back later^ 

 but anteriorly it is involved in the asymmetry which is such 

 a marked feature of the larva, and will be considered in the 

 chapter on the general development. 



The first indication of the fjiture atrial cavity appears in 



