xiv CHAETOGNATHA 435 



maturity reaches and fuses with the ectoderm at a point where the 

 female opening is formed. Buchner (1910) has found that the large 

 ova pass into the narrow oviduct through narrow, slit -like openings 

 which are only visible at the moment of passage. From the descrip- 

 tion just given it follows that the male ducts are of ectodermal 

 origin, but that the origin of the female ducts is doubtful, for the 

 precise origin of the loose cells which give rise to them has not been 

 ascertained. 



AFFINITIES OF THE CHAETOGNATHA 



As mentioned in the beginning, the affinities of the Chaetognatha 

 are very obscure. The early development so far, the segmentation 

 of the egg, the formation of the archenteron by invagination, and the 

 formation of the coelom as lobes of the archenteron, is of a very 

 primitive type and has been compared to that of Brachiopoda and 

 Echinodermata. What justice there may be in the latter com- 

 parison will be considered when the development of Echinodermata 

 is described. 



With regard to Brachiopoda, Conklin lays stress on the difference 

 between a single anterior septum dividing gut from coelom in 

 Brachiopoda and the two lateral folds in Chaetognatha, but this 

 difference is annulled if, as Kowalevsky describes, the coelom in the 

 Brachiopod Argiope is separated from the gut by two lateral septa. 

 But the posterior position of the last traces of the blastopore, and 

 the early appearance of the stomodaeum, are two great differences in 

 development which separate Chaetognatha from Brachiopoda. 



Another great difference is the total absence of cilia in the 

 embryo of Sagitta. This is probably partly correlated with the fact 

 that the development of Sagitta, in spite of its primitive features, 

 must be extremely shortened, for the larva, though it differs from the 

 adult in its functionless alimentary canal and solid mesoderm, 

 nevertheless has the same mode of life as the adult. On the other 

 hand, in Brachiopoda, as indeed in nearly all the groups of the 

 animal kingdom which we have as yet studied, the larva has quite a 

 different mode of life and different locomotor organs from the adult. 

 The very early differentiation of the genital cells is another sign of 

 shortened development. 



Hertwig compared the Chaetognatha to an Annelid with three 

 segments, and a comparison has also been suggested between the three 

 pairs of body-cavities in Chaetognatha and the three segments of the 

 Brachiopod larva. But with whatever justice such a comparison can 

 be applied to the head and trunk segments, it is doubtful, as Doncaster 

 wisely remarks, how far this explanation can be applied to the 

 separation of trunk and tail segments ; for this latter separation occurs 

 in Chaetognatha long after the separation of the head-cavities, and it 

 appears to be a device to separate male and female reproductive organs. 



Giinther (1907) has written a long paper to try to prove that 

 Sagitta is related to the stem of the Mollusca. As the parent stems 



