INTRODUCTION. 5 
make an extemporized grate and boil a kettle in the 
wood, when a much more efficient grate, full of lighted 
coals, is already boiling some other kettle at home?); 
and if they somewhat unduly prolong a research, the 
full meaning of life is, after all, not exhausted by the 
experiences of a mill-horse, and it is well to remem- 
ber that so soon as we cease to take pleasure in our 
work, we are most likely sacrificing one part of our 
humanity to the altar of some other, and probably 
less worthy, constituent. 
I may now say a few words on the scope of the 
investigations which are to be described in the 
present treatise. To some extent this is conveyed 
by the title ; but I may observe that, as the “ primi- 
tive nervous systems” whose physiology I have 
sought to advance are mainly subservient to the 
office of locomotion, in my Royal Society papers 
upon these researches I have adopted the titie of 
“Observations on the Locomotor System” of each 
of the classes of animals in question. It is of 
interest to notice in this connection that the plan 
or mechanism of locomotion is completely different 
in the two classes, and that in the case of each class 
the plan or mechanism is unique, 2. is not to be 
met with elsewhere in the animal kingdom. It is 
curious, however, that, in the case of one family of 
star-fish (the Comatule), owing to an extreme 
modification of form and function presented by the 
constituent parts of the locomotor organs, the 
method of progression has come closely to resemble 
that which is characteristic of jelly-fish. 
- There is still one preliminary topic on which I 
