CHAPTER I 

 ORIGIN AND RADIATION 



I n the state of British Columbia there are certain rocks, the Burgess 

 Shales, which were laid down over four hundred million years 

 ago. These rocks are remarkable for the fossils which they contain. 

 Some of these fossils look very similar to Crustacea which are alive 

 to-day. The age of the Burgess Shales takes us back to within a 

 comparatively few million years from the beginning of the detailed 

 fossil record. This means that we cannot expect to get much 

 information from fossils about the actual origin of the Crustacea, 

 for the presence of these highly organised creatures so near to the 

 beginning of the fossil record indicates that many features of the 

 crustacean type of organisation were already in existence. But first 

 we have the problem of whether these early fossils were, or were 

 not Crustacea. 



A satisfactory definition to include all Crustacea is extraordin- 

 arily hard to frame, because so many of them have become highly 

 modified, particularly those which are parasitic. However, most 

 Crustacea have, at least at some stage, two pairs of appendages in 

 front of the mouth, which are called the antennules and the 

 antennae. Generally too there are at least two or three pairs of 

 appendages near to the mouth which have special functions in 

 feeding. The first of these appendages are the mandibles, which 

 function in the same manner as jaws with teeth, but working from 

 side to side instead of up and down. Two pairs of maxillae lie 

 behind the mandibles and are usually concerned in pushing food 

 into the mouth. Now if we try to study these appendages in the 

 early fossils we meet an insuperable obstacle. Although many of 

 these fossils are beautifully preserved they do not provide us with 

 the necessary details concerning the mouthparts — if indeed they 

 ever had any mouthparts ! 



For this and other reasons many authorities prefer to include all 

 the Burgess Shale fossils which look like Crustacea in a group called 

 the Pseudocrustacea, and to ally this group with those remarkably 

 abundant fossils the trilobites. 



