150 SOME FURTHER RESEARCHES 



which it is to be noted is pigmentary and vascular, having received 

 no distinctive designation that would indicate its homologies. 



Now, it may be objected to this that the term epidermis is 

 applied in Vertebrates to the external cellular epithelium of the 

 body, and that the term cuticle, employed by Gegenbaur, is the 

 more correct to apply to a homogeneous secreted covering. At the 

 same time, it must not be overlooked that the subjacent cellular 

 layer in the Earthworm and in Limnodrilus is also vascular, a fact 

 which goes far to remove it from the class of epithelial structures, 

 and should make us rather regard it as the homologue of the 

 dermis of the higher animals. In which case the external struc- 

 tureless layer is, so far as position goes, truly an epidermis, but 

 it is difficult so to regard it without admitting that it is not a 

 secretion, but that it is of cellular origin,^ which seems quite 

 opposed, not only to my observations, no trace of cells being 

 discoverable in it, but also to the statement of Gegenbaur, who 

 says that in some of the annelids, pore-canals, so distinctive of 

 the cuticular structures of insects, may be seen in it. Moreover, 

 I am not quite sure whether certain mucous pores, which 

 Lankester afterwards describes in the Earthworm, are not of the 

 nature of pore-canals. 



The subject seems to raise the whole question of the origin of 

 the cuticular layers of the Annulosa, and a further question 

 appears to arise with respect to the annelids, viz., what provision 

 exists for the growth of the structureless layer (if absolutely 

 structureless it be), which we find in them. Insects and Crustacea 

 have a provision in their periodical ecdysis for a growth of their 

 coverings, to keep pace with that of the body, but no such pro- 

 vision exists in the worms. The cuticular coverings of the latter, 

 however, are not to be compared for hardness and rigidity with 

 those of the former, and where it is very soft, the growth of the 

 cuticle may be, perhaps, compared to that of the cell-wall keeping 

 pace with that of its contents. In some worms, however, the 

 cuticle is too thick and consistent to allow us to suppose it to 

 enlarge in this way ; nor do I quite think it is the case with 



* Lowne (Anat. Blow Fly, p. lo) ascribes a cellular origin to the "cuticle 

 or epidermis " of insects, saying distinctly that it is formed of " coalesced 

 cells." 



