314 THE ZOOLOGIST. 
Here the cells of the epithelial layer underlying the structureless 
cuticle which they form is, as Prof. Ray Lankester has shown, 
penetrated by fine blood-vessels, which form an intra-epithelial 
system, and come as near to the surface as they possibly can. 
More recently M. Stole has described an elaborate system of 
plexuses in the integument of an earthworm, Ilyodrilus, and 
a system of superficial capillaries has been found in the earth- 
worms Pericheta and Perionyx by our prosector, Mr. Beddard. 
As our knowledge of the finer anatomy of lower forms increases 
we shall, I believe, be able to add considerably to the list of 
animals which, with a vague respiration, have their oxygenating 
blood-vessels brought into the closest possible relation with the 
external medium. 
The greater number of definite respiratory organs fall under 
the head of outpushings or ingrowths; the former are called 
gills, the latter air-tubes (trachez) or lungs. 
2. The system of respiration by gills is preceded physio- 
logically by the method adopted by the Starfish ; when carried 
further and made a definite arrangement, it often consists in the 
possession of a number of delicate tubes, hollow within, and 
having thin walls; within flows the respiratory blood or fluid, 
without is the external respiratory medium, the movement of 
which is often aided by the presence of those delicate proto- 
plasmic processes which we call cilia; the constant contraction 
of these gives rise to the formation of currents in the water, and 
so allows of a renewal of the oxygen-carrying water. Delicate 
respiratory filaments of this kind are to be found among some of 
the bivalved Mollusca, such as the Sea-mussel (Mytilus), or the 
Scollop (Pecten); but they become converted into continuous 
plates in the fresh-water Mussel (Unio), and a large number of 
Mollusca. Here the motive power of the respiratory medium is 
to be found in the cilia which cover the gill-filaments or the 
plates into which they fuse. In the Crayfish, where the gills 
form plumes on either side of the body, the cells of which they 
are made up are, like all other cells in the bodies of Crustacea, 
quite devoid of cilia; the motive-power must. consequently be 
looked for elsewhere. First of all we may find it in the fact that 
some of the gills are attached to the bases of the walking limbs, 
and it is clear that when these limbs move the gills will move 
too, and so agitate the water that bathes them; but even when 
