ON THE DIPLOOHORDA. 



327 



appears to be destined to play a very secondary part in the 

 higher Coelomata in any case. In animals which take up a 

 sedentary existence it is no longer required to any great extent, 



9. 10. 



11. 



9. Ventral view of Stage II. 10. Dorsal view of ditto. 11. Lateral 



view of ditto. 

 for the " animal" functions are at a low ebb. Again, in those 

 which do not thus retire from the arena of pelagic or littoral 

 life it is replaced by the shifting forwards of the "metameric" 

 segments from behind. 



The cavities (2, 2') of the two lateral segments are the most 

 suited, by their position, for performing the functions of 

 digestion and excretion ; and at the same time their position 

 immediately behind the mouth causes these segments to be 

 adapted (by tentacles, &c.) for ingestive functions. They may 

 be termed the '' mesomeres/' and are recognised in Hemi- 

 chorda as the collar-segment. The last (3) or post-anal is 

 only found in this position in rare cases, for the next stage 

 is soon reached. 



The nerve-ganglion is seen to be making its way forwards 



