170 THE APODID^ PART i 



GENERAL CONCLUSION BASED UPON THE ARGU- 

 MENTS CONTAINED IN THE FOREGOING SECTIONS. 



In pre-Cambrian times, of which there arc now 

 no fossil remains, a browsing carnivorous Annelid ac- 

 quired the habit of keeping its " head," i.e., its first five 

 segments, bent round so that the mouth faced ventrally 

 and posteriorly, and used its parapodia for pushing 

 food into its mouth. The antennae, antennal para- 

 podia, and parapodia developed gradually into Crus- 

 tacean antennae, mandibles, maxillae, and limbs. For 

 the protection of the exposed anterior dorsal surface, 

 a shield, to be more accurately described later on, 

 was developed out of a fold of the tergum of the fifth 

 segment, the posterior edge of which grew perhaps as 

 a carrier of defensive thorns. At the posterior end 

 of the body, the inherited number of Annelidan 

 segments gradually ceased to be developed, and 

 remained in a rudimentary or larval condition. 

 The gradual development of a thickened cuticle led 

 to transformations of outer and inner organisation 

 sufficient to change the Annelidan into the Crustacean 

 type. The modern representative of this Crustacean- 

 Annelid is Apus. 



We have now to see if it is possible to deduce 

 the principal groups of both living and extinct 

 Crustacea either from this racial form or from a 

 similar Crustacean-Annelid. This is clearly the best 

 test of the truth of the morphological and anatomical 

 reasoning contained in this first part. 



