360 R. T. GUNTHhlU. 



We shall endeavour to meet these objections, and to show 

 that the resemblances are shared by all the chief organs in the 

 Chaetognath anatomy, namely, by the nervous, alimentai'y, 

 genital, and other systems; that also in their development 

 the ChfBtognath approaches very close to the primitive 

 Molluscau type, and that in the case of those features in 

 which the Chtetognath organisation appears not to be of the 

 MoUuscan type, interesting parallels are to be found within 

 the limits of the Molluscau phylum itself. 



At the same time it is not proposed to merge the Chgeto- 

 gnatha in any one of the existing Molluscan classes; they 

 are rather to be regarded as modern and modified repre- 

 sentatives of a free-swimming ancestor from which the 

 Molluscan phylum has sprung. They represent, in fact, 

 ArchimoUusca modified for an active swimming life in the 

 open waters of the ocean, with affinities to all existing 

 classes of the phylum. They have, to our mind, more 

 primitive features than the '' Archimollusc" of Lankester, 

 which we shall term the creeping Archimollusc to distinguish 

 it from our swimming type of archimollusc; and to emplia- 

 size this relationship we propose to consider the Clitetognatha 

 as belonging to the lowest class in the Molluscan phylum 

 from which the others have sprung. 



External Organisation. 



The external configuration of the Chsetognath bod}' has 

 justly earned for them their common name of "Arrow- 

 worms.^' Human ingenuity has not got further in striving 

 after the form best adapted to fast swinnning beneath the 

 surface of the water. The lines of the Sagitta and the 

 Whitehead torpedo are the same. Comparatively few of the 

 Mollusca, which have taken to a swimming mode of life, 

 have reached such perfection of outline; in all the diffi- 

 culty of disencumbering themselves of unwieldy shells and 

 feet and mantle flaps has had to be overcome, and even 

 when these have all gone, as in the languid Phyllirhoe, the 



