216 STUDIES IN EVOLUTION 



taken as of phylogenetic value and indicative of the primitive 

 type of limb structure. 



The whole series of endopodites anterior to the last two 

 or three show modifications from the phyllopodous type, the 

 change involving progressively from one to all of the endites. 

 The endopodites of the pygidium have a true phyllopodiform 

 structure, and are composed of broad leaf-like joints, wider 

 than long. This character is gradually lost in passing ante- 

 riorly, the distal endites being the ones first affected. By 

 the time the anterior pygidial limb is reached, the three 

 distal joints are longitudinally cylindrical. The ninth tho- 

 racic endopodite shows a fourth endite becoming cylindrical, 

 and on the first and second thoracic legs even the proximal 

 ones are thus modified, making all the endites of these limbs 

 slender in form. 



This gradual modification of a phyllopodiform swimming 

 member into a long, jointed, cylindrical, crawling leg de- 

 serves more than passing notice, for here, probably, better 

 than in any known recent form, can the process and its signifi- 

 cance be studied. No living type of crustacean more nearly 

 conforms to the theoretical archetype of the class than do the 

 trilobites, and as Triarthrus belongs to an ancient Cambrian 

 family, it may be expected to retain very primitive characters. 



In this genus several causes evidently influenced the modi- 

 fication of the appendages. First may be mentioned the 

 specialization into oral organs of the gnathobases of the head, 

 which would tend toward a reduction of the other portions 

 of the limbs. Next, the assumption of a walking habit would 

 gradually lead to a corresponding adaptation of the anterior 

 thoracic endopodites, this region of the body being naturally 

 the place where they would be most operative. Lastly, any 

 tendency to change the form of the anterior limbs would 

 be accelerated through the greater number of moults they 

 undergo as compared with the abdominal appendages. 



Since the anal segment of Crustacea contains the formative 

 elements out of which all the trunk segments are successively 

 developed, it may be considered as the same segment in all 



