ON THE DEVKLOPMENT OF SAGITTA. 393 



tlie innermost cells arrange themselves to form an epi- 

 thelium. 



The differences between the two groups are, however, very 

 wide ; in the first place, the body-cavity of the Nematoda is 

 not certainly coelomic, and the intestine is not supported by 

 mesenteries, nor are there transverse septa. The nervous 

 systems of the two phyla are also very distinct, and when all 

 the points of resemblance and difference are considered 

 together it is evident that there can be no near relationship 

 between them. The conclusion, therefore, must be that if 

 the nearest connections of the Chaetognatha are with the 

 Nematodes, yet the two groups have diverged very widely 

 owing to a difference of habit : the Nematoda were perhaps 

 primitive Coelomata which have become degenerate through 

 parasitism ; while the ChaDtognatha, if they branched off from 

 the same original stock, have become fundamentally modified 

 for pelagic existence. In the present state of our knowledge 

 it seems safest to regard the Chgetognatha as descended from 

 a primitive ccelomate stock, from which the Annelida have 

 arisen on the one hand ; while, on the other, the Nematoda 

 probably branched off, but lost many of their original 

 characters owing to their parasitic habit. 



APPENDIX. 



On the Anatomy of Sagitta minima. 



Grassi, in his general account of the anatomy of the group, 

 states that this species has irregular septa between the 

 alimentary canal and the body-wall, but admits that he has 

 not been able to throw any light on them by sections. 



These septa are very conspicuous in the adult S. minima, 

 and form the readiest means of identifying it at a glance. 

 They occur especially in the anterior part of the body, but at 

 intervals through the whole trunk region ; they are quite 



