278 THE PROTOCHORDATA. 



head-cavities of Amphioxus, although they have a some- 

 what different origin. 



It should not be forgotten that Sagitta occupies a very 

 isolated position in the zoological system, being placed in 

 a group by itself, the ChcBtognatha, and that therefore the 

 peculiarities of its organisation cannot be taken as repre- 

 senting any definite intermediate stage in the phylogeny 

 of other forms, yet, from a general standpoint, the con- 

 ditions which it presents in its life-history are highly 

 instructive. 



The head-cavities of Sagitta arise by constriction from 

 the anterior extremities of the single pair of archenteric 

 pouches which give rise to the coelom of the adult. They 

 remain distinct and separate on either side of the head 

 throughout life. If, now, we imagine them to grow for- 

 ward and fuse together in front of the mouth, in a simi- 

 lar manner to that described above for the enterocoelic 

 pouches of Asterias, we should have a praeoral body-cavity 

 of a similar character to that of Balanoglossus. 



Now, the ultimate position of the mouth under these 

 new conditions would depend upon circumstances affect- 

 ing the whole organisation of the animal. 



In an animal whose grade of organisation was on an 

 approximate level with that of Sagitta the mouth would 

 undoubtedly remain on the ventral side of the body. But 

 in an animal whose organisation had reached the stage 

 of evolution represented by that unknown ancestor of 

 Amphioxus (most nearly represented at the present time 

 by the Ascidian tadpole), whose notochord did not extend 

 beyond the anterior limit of the neural tube, the mouth 

 would pass to the dorsal side of the body to come into 

 connexion with the neural canal. 



