364 R. T. GiJNTHER. 



effect of time, its loss has no doubt also been accelerated by 

 the marvellous rapidity with which Clifetognath development 

 takes place. 



Segmentation. 



Although the body cavity appears to be divided into 

 three paired cavities by septa, there is no justification 

 for regarding the Chaetognatha as exhibiting any real 

 raetameric segmentation at all. In the first place the two 

 transverse septa cannot be regarded as homologous with 

 one another, or with the septa which separate the meta- 

 meres of Annelida, because in time, mode, and purpose 

 of origin they are absolutely different. Embryological 

 examination shows that the anterior septum is produced by 

 the meeting and fusion of the somatic and splanchnic meso- 

 blast very early in embryonic life, at a time when differentia- 

 tion of tissues has not begun and the mesoblast is still 

 continuous with the hypoblast ; and that the posterior septum 

 appears in close connection with the genital cells, and is not 

 formed by the whole thickness of the mesoderm, but probably 

 only by the cellular envelopes of the genital cells, and cer- 

 tainly by the splanchnic layer exclusively. 



In the light of our new knowledge concerning these 

 developmental phenomena, the theory of a close relationship 

 of Chgetognatha to an unsegmented group of animals of a 

 high grade of organisa'tion is in no way prejudiced by the 

 division of the body into three sections which have the guise 

 but not the reality of metameres. 



Integument. 



The Chaetognatha differ from the majority of MoUusca in 

 the absence of a more or less extensive ciliated epithelial 

 covering of the body both in the larval and adult stages. But 

 though devoid of a general investment of cilia which might 



