202 PALEONTOLOGY 



process (gnathobase) projects towards the middle line. 

 In the outward direction two branches spring from the 

 coxopodite (hence the appendages are said to be biramous) : 

 the inner one, or endopodite, consists of five movable 

 joints, not unlike one of the ordinary legs of a crab ; the 

 outer, exopodite, consists of one long joint followed by a 

 great number of very short joints, the whole fringed with 

 numerous hair-like setce. Evidently the endopodite is 

 adapted for crawling on the bottom, the exopodite for 

 swimming through the water. The function of the 

 gnathobase is known from the habits of modern 

 Crustacea with very similar appendages. The right and 

 left gnathobases in any somite approach one another to 

 grasp any solid body between and pass it forward to the 

 pair in front, and so on until it reaches the mouth, where 

 it is swallowed or rejected according to its being edible 

 or not. 



Thus the appendages of Triarthrus perform the three 

 functions of crawling, swimming, and seizing food, as do 

 those of such a Crustacean as Apns : to these may be 

 added probably that of respiration, for they present a 

 large surface of thin cuticle to the sea- water in a region 

 where it must be constantly agitated by the movements 

 of the limbs. In more specialized trilobites we may 

 expect a greater division of labour some appendages 

 will be specialized as jaws, others for crawling, and 

 others for swimming (unless one of these two means of 

 locomotion is lost altogether), and definite gills will 

 appear as outgrowths from some appendages instead of 

 the whole surface being used for respiration. In Calymene 



