50 PRIMITIVE ANIMALS 



(Fig. 9), found in the oldest stratified rocks, which 

 undoubtedly represent very generalized and primitive 

 Crustacea, but they do not form a transition into any 

 of the other Arthropodan classes. It is only of recent 

 years that the relationship of the Trilobites has been 

 definitely established, but the discovery that the 

 anterior head appendages were antenniform, and the 

 succeeding limbs biramous, i.e. consisting of a basal 

 portion carrying two terminal branches, placed their 

 Crustacean affinity beyond doubt. The possession of 

 the biramous limb is the most characteristic feature 

 of the Crustacean class ; despite the wonderful variety 

 exhibited by Crustacean limbs, from the pincers of 

 the crab or lobster to the complicated leaf-like paddles 

 of the water-flea or brine-shrimp, from the tooth-like 

 mandible to the tactile second antennae, all the 

 appendages of the Crustacea can be reduced to the 

 biramous plan, with the exception of the first pair of 

 antennae, and it is interesting to know that even in 

 the Trilobites, where all the limbs are in their most 

 primitive and least specialized condition, the first 

 antennae are sui generis and of a simple uniramous 

 structure. It may be truly said that the recent 

 discovery of the nature of the appendages in the 

 Trilobites is a powerful vindication of the compara- 

 tive methods of morphology, so far as the Crustacean 

 limb is concerned. 



Within the Crustacean class we meet with two 



