ON THE DEVELOPMENT OF >SA(ilTTA. 391 



mesoderm differs greatly from that of the trunk, and in an 

 animal which develops so rapidly as Sagitta it seems natural 

 that the separation should be early and complete. 



If this theory be correct it destroys the chief ground for 

 associating the Chajtognatha with the Annelida, for the three 

 segments of the body would not be homologous with one 

 another, and therefore cannot be compared with Annelid 

 metameres. Such a separation seems justified by a closer 

 examination of the other points of resemblance, the chief of 

 which are the similarity of the body-cavity and muscles and 

 the comparison of the reproductive ducts with nephridia. 

 The longitudinal muscles of Sagitta certainly have a close 

 similarity with those of Annelids both in structure and 

 arrangement, but from the arrangement of the nuclei which 

 border the coelom in connection with the muscles it seems 

 probable that there is in reality no coelomic epithelium in the 

 Cheetognatha, but that the body-cavity is bordered by a 

 single layer of cells, the ends of which next to the cavity 

 remain protoplasmic, while the deeper ends develop into 

 muscle-fibres. Such an arrangement exists in the Nematoda, 

 but it differs greatly from the Annelid plan of a definite 

 coelomic epithelium overlying the longitudinal muscles. 

 Further work on the histology of the group is required 

 before this point can be finally settled. 



The relation of the genital ducts to nephridia can be dealt 

 with more certainly, and this also gives evidence against the 

 Annelid theory. In the first place, there is no trace of 

 genital ducts in the early stages, and even in well-grown 

 young they are quite absent until maturity approaches. In 

 this point they differ greatly from nephridia, which appear 

 at an early stage. In their mode of development they differ 

 no less, for, as was shown above, the male ducts arise as 

 epidermal thickenings, and are formed as splits in the 

 ectoderm for the greater part, if not the whole, of their 

 length, while true nephridia are always chiefly mesodermal 

 in origin. 



The development of the oviducts is less certain, Imt lend 



