54 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



the general, if not universal, presence of a system of " am- 

 bulacral vessels" consisting of a circular canal around the 

 mouth, whence canals usually arise and follow the middle line 

 of each of the ambulacral metameres. And, in the typical 

 Echinoderm, these canals give off prolongations which enter 

 certain diverticula of the body-wall, the pedicels or suckers. 



All Echinoderms have a calcareous endoskeleton. 



In the chapter allotted to these animals, it will be shown 

 that they are modifications of the Turbellarian type, brought 

 about by a singular series of changes undergone by the endo- 

 derm and mesoderm of the larva or Echinopoedium. 



III. THE PHYSIOLOGICAL DIFFERENTIATION OF ANIMALS, AND 



THE MORPHOLOGICAL DIFFERENTIATION OF THEIR ORGANS. 



Regarded as machines for doing certain kinds of work, 

 animals differ from one another in the extent to which this 

 work is subdivided. Each subordinate group of actions or 

 functions is allotted to a particular portion of the body, which 

 thus becomes the organ of those functions ; and the extent 

 to which this division of physiological labor is carried differs 

 in degree within the limits of each common plan, and is the 

 chief cause of the diversity in the working out of the common 

 plan of a group exhibited by its members. Moreover, there 

 are certain types which never attain the same degree of physi- 

 ological differentiation as others do. 



Thus, some of the Protozoa attain a grade of physiological 

 complexity as high as that which is reached by the lower Me- 

 tazoa. And, notwithstanding the multiplicity of its parts, no 

 Echinoderm is so highly differentiated a physiological ma- 

 chine as is a snail. 



A mill with ten pairs of millstones need not be a more 

 complicated machine than a mill with one pair ; but if a mill 

 have two pairs of millstones, one for coarse and one for fine 

 grinding, so arranged that the substance ground passes from 

 one to the other, then it is a more complicated machine a 

 machine of higher order than that with ten pairs of similar 

 grindstones. In other words, it is not mere multiplication of 

 organs which constitutes physiological differentiation ; but 

 the multiplication of organs for different functions in the first 

 place, and the degree in which they are coordinated, so as to 

 work to a common end, in the second place. Thus, a lobster 

 is a higher animal, from a physiological point of view, than a 



