436 INVEETEBEATA CHAP, xiv 



of Mollusca and of Annelida are identical, this conclusion is not so 

 different from that of Hertwig as might at first sight appear to be the 

 case. But Giinther's conclusions are based entirely on interpretations 

 of adult anatomy and ignore the question of development, in which 

 Sagitta differs absolutely from any Mollusc so far studied. The facts 

 that Sagitta is essentially a chitinous animal which nowhere develops 

 cilia, and that its locomotor organs are both developments of this 

 cuticle, constitute a deep gulf between it and any of the soft- 

 skinned Mollusca. 



It has been suggested that the Nematoda have affinities with the 

 Chaetognatha. The arguments for and against this will be considered 

 in the next chapter, when the development of Nematoda will be 

 described. But even if we should finally decide that such an affinity 

 exists, it would throw no light on the origin of Chaetognatha, because 

 in that case the Nematoda would have to be regarded as much 

 modified and degenerate Chaetognatha which had taken to burrowing 

 and parasitic habits. 



The conclusion to which we are led, the arguments for which will 

 be developed later, is, that in immensely remote times there existed 

 a stock of free-swimming animals in which the coelom still was in 

 open communication with the gut. This hypothetical stock may be 

 called the Protocoelomata, and from it all the groups of coelornate 

 animals would be descended. If this be correct, the Chaetognatha 

 would represent a small, comparatively unmodified offshoot from 

 that stock, a group which has been able to hold its own until the 

 present day. 



LITERATURE REFERRED TO 



Buchner, P. Die Schicksale des Keimplasmas der Sagitten in Reifung, Befnichtung, 

 Keimbahn, Ovogenese und Spermatogenese. Festschr. zum 60ten Geburtst. Hertwigs, 

 vol. 1, 1910. 



Biitschli. Zur Entwicklungsgeschichte der Sagitta. Zeitschr. Wiss. Zoo!., vol. 

 23, 1873. 



Doncaster, L. On the Development of Sagitta, with Notes on the Anatomy of the 

 Adult. Quart. Journ. Micr. Sc., vol. 46, 1902. 



Gunther, R. The Chaetognatha or Primitive Mollusca. Ibid., vol. 51, 1907. 



Hertwig, 0. Die Chaetognathen. JenaZeit, vol. 14, 1880. 



Kowalevsky. Embryologische Studien an Wiirmern und Arthropoden. Mem. 

 Acad. Petersbourg, 7th series, vol. 16, 1871. 



