ON THK DEVELOPMENT OF SAGITTA. 387 



lumen and becomes definitely supported by dorsal and ventral 

 mesenteries^ and the anus is formed just in front of the pos- 

 terior transverse septum. 



The remaining changes concern chiefly the reproductive 

 organs ; the genital cells divide to form four groups of cells, 

 and these grow into cylindrical masses, and form the young 

 ovaries and testes. The latter give off groups of cells into 

 the tail cavities, and from them the sperm-cells arise, while 

 the male genital ducts and seminal vesicles arise by thicken- 

 ing and splitting of the lateral ectoderm of the tail. 



The ovaries grow forward as a cylindrical mass of cells, and 

 as maturity approaches the oviducts appear along the outer 

 edges, but completely enclosed in ovarian cells. They seem, 

 therefore, to be formed actually as a cavity in the ovary, and 

 not by ectodermal invagination as has been supposed. 



It now remains to consider what bearing these facts have 

 upon the systematic position of the Chjetognatha. There 

 can be no doubt that the group is an exceedingly isolated one, 

 and the fact that all the members of it are closely related to 

 one another, and the absence of any skeleton which could be 

 traced back in palseontology, make it peculiarly difficult to 

 determine its relationships with certainty. The Chaetognatha 

 have no close resemblances with any other group, and it is 

 frequently found that when judged by different chai'acters 

 their nearest relationships appear in quite different directions, 

 and in consequence they have been associated by various 

 authorities with most of the phyla of the animal kingdom. 



In discussing their position, since their important charac- 

 teristics offer so little help, it is necessary to descend to minor 

 characters and details, and therefore it is impossible to arrive 

 at an absolute certainty, but it is at least possible to discover 

 in what directions the probability of relationship lies. The 

 Cha3tognatha have been associated, on various grounds, with 

 many different groups, but it will not be necessary to discuss 

 the merits of all. By the majority of authorities on the 

 subject they have been placed either with the Annelida or 

 with the Nematodes, but by some the threefold division of 



