CATEGORIES OF CLASSIFICATION. 39 



distinction, the difference between classes and 

 orders cannot be understood ; for classes and 

 orders rest upon a just appreciation of these two 

 categories, which are quite distinct from each 

 other, and have by no means the same signifi- 

 cance. 



Again, quite distinct from both of these is the 

 character of form, not to be confounded either 

 with complication of structure, on which orders 

 are based, or with the execution of the plan, on 

 which classes rest. An example will show that 

 form is no guide for the determination of classes 

 or orders. Take, for instance, a Beche-de-Mer, 

 a member of the highest class of Radiates, and 

 compare it with a Worm. They are both long 

 cylindrical bodies ; but one has parallel divisions 

 along the length of the body, the other has the 

 body divided by transverse rings. Though in 

 external form they resemble each other, the one 

 is a worm-like Radiate, the other is a worm-like 

 Articulate, each having the structure of its own 

 type ; so that they do not even belong to the 

 same great division of the animal kingdom, much 

 less to the same class. We have a similar in- 

 stance in the Whales and Fishes, the Whales 

 having been for a long time considered as Fishes, 

 on account of their form, while their structural 

 complication shows them to be a low order of the 

 class of Mammalia, to which we ourselves belong, 



