CONCLUSION. 453 



typical structure of the ventral nerve cord in primitive arthropods. Whether or 

 no it consists of distinct neuromeres, secondarily united, will probably be mani- 

 fest on a more careful examination of its transverse commissures. 



Conclusion. The chaetognatha are unquestionably primitive arthropods, 

 somewhat degenerate. They are adapted to a permanent pelagic existence and 

 reach sexual maturity without passing much beyond the nauplius stage. Their 

 relation to the arthropods is shown by the presence of a typical teloccele; ccelomic 

 pouches; the early appearance and the location of the sex cells in the segmenting 

 egg; the division of the body into head, thorax, and abdominal regions; the loca- 

 tion of the ovaries in the thoracic region, and the testis in the abdominal region of 

 the adult; the character of the jaws; the lateral or pleural folds; the bivalve mantle 

 folds of the head (prepuce); the prevalence of chiten; the fibro-cartilaginous plate 

 in the jaw muscles (endocranium) ; the structure of the brain and the arrangement 

 of stomodaeal nervous system; and finally the structure and distribution of the 

 lateral and parietal eyes and " frontal organs." 



The history of the chaetognatha is profoundly significant, for it shows us a 

 group of animals that combines in a convincing manner some of the important 

 anatomical characters of adult arthropods, such as those above mentioned, with 

 the more striking embryonic characters of typical acraniates, such as the telopore 

 and teloccele and ccelomic pouches. Their structure and development justifies 

 the conclusions already reached in other ways, that the teloccele and ccelomic 

 pouches are secondary, not primary characters, and that they have been acquired 

 from arthropod ancestors, partly as a result of degeneration, loss of yolk, and a 

 consequent rapid process of early development. 



The chaetognatha resemble the cephalochorda and the enteropneusta in their 

 straight, elongated body and intestine, but differ from them in the presence of an 

 open neurostoma and the absence of a haemostoma and gill clefts. They are 

 definitely excluded from the nematodes, annelids, molluscs, and rotifers with 

 which they have been affiliated by various authors by the absence of the gastrula 

 and trochosphere stages, by their highly modified development, as well as by the 

 peculiar structure of their brain, stomodaeal ganglia, and cephalic sense organs. 

 They are clearly acraniates, but are not closely affiliated with any other sub- 

 division of the group. 



