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 title 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, i.e. 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 Comatulce), 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 



