THE AGE OF INVERTEBRATES OF THE SEA. 79 



Mesozoic, though the cuttles as a whole perhaps culminate in 

 the modern. 



The remarkable group of the Trilobites had precedence in 

 order of time of the Nautiloid shell-fishes. No animal structures 

 can well be more dissimilar than those of the two great groups 

 of aquatic animals which popular speech confounds under the 

 name of " shell-fishes." Take a whelk and a crab, for example, 

 and compare their general forms, ^he structure of their shells, 

 and their organs of motion, and it is scarcely possible to imagine 

 any two animals more unlike; and when we examine their 



b 



a, Paradox ides. 



Fig. 75.— Cambrian Trilobites. 



d, Dikellocephalus. c, Conocephalites (head), d, AgnosUts (head 

 and tail). 



anatomy in detail this difference does not diminish. They have, 

 it is true, corresponding parts, and these parts serve similar 

 uses, but in plan of structure they are wholly different. Yet 

 both animals may live in the same pool, and may subsist on 

 nearly the same food. If we attempt to find some common 

 type which both resemble, we may trace the structure of the 

 crab back to those of some of the marine worms with which it 

 has some affinity, and those of the whelk to such creatures as 

 the Lingular which are supposed to have a resemblance to 



