THE ohj;tognatha, or primitive mollusoa. 363 



conchyolinous substance within the fold either in the form of 

 fin-rays or of a continuous lamina. 



If we may be permitted to imagine the alterations of 

 structure which a typical Mollusc would undergo if it were to 

 become adapted for an active, free-swimming, predacious 

 life ; I think that, judging by living examples, we should 

 find that they would be in the direction of the structure of 

 the Chsetognatha. 



That foot, visceral hump, shell, and mantle cavity might 

 disappear, is clearly shown by the structure of Phyllirhoe. 

 But the Chaetognatha differ from Phyllirhoe in their smaller 

 size, and in the far greater activity of their swimming move- 

 ments. To these two factors we attribute the relatively large 

 development of certain organs, such as the longitudinal 

 muscular system and the non-existence of others, e.g. separate 

 vascular and respiratory systems respectively. 



It may be premised at once that there is nothing in the 

 Chfetognath anatomy or development which we have felt able 

 to homologise with the shell or shell-gland of the Mollusca. 

 We cannot, however, consider that the absence of this pallial 

 organ is very material to the main issue, for even among 

 Mollusca of very moderate swimming powers the shell is 

 usually the first structure to dwindle and disappear (Hetero- 

 pods, Phyllii'hoe, etc.), and might therefore be expected to be 

 altogether absent in a Mollusc built upon such perfect lines 

 for swimming as Sagitta. But it may be argued, if the 

 ancestor of Chgetoguatha had a typical MoUuscan shell, surely 

 some trace of a shell-gland would be noticeable in the younger 

 stages of their growth, it being so conspicuous a feature in 

 Molluscan developments. For its absence two explanations 

 may be given : firstly, by our hypothesis, the Chsetognatha 

 are presumed to have become modified for their pelagic life 

 at a very early period, before the Molluscan phylum and 

 the Molluscan shell had been in existence for a long time, in 

 other words, the Chsetognatha are a far older group than any 

 of the shell-less Mollusca. 



If the loss of all trace of the shell-gland may have been the 



