186 STUDIES IN EVOLUTION 



including the neck ring. These may therefore be taken as 

 representing, in so far, the original segmentation of the cepha- 

 lon, and agree with what is generally accepted as the primitive 

 structure in modern true Crustacea. The head portion of the 

 protaspis clearly shows this pentasomitic structure, and evi- 

 dently carried a corresponding number of paired limbs on the 

 ventral side. It has also been demonstrated that the annula- 

 tions on the axis of the pygidium correspond to the number 

 of paired limbs beneath, exclusive, of course, of the anal seg- 

 ment. Here, too, it is possible to tell from the jsygidial por- 

 tion of the protaspis the number of limbs present during life. 

 The protaspis of Triarthrus, represented in Plate III, figure 

 13, on this basis had five pairs of limbs attached to the head 

 portion, and two pairs to the pygidium. 



Next, as to the composition and form of these elementary 

 protaspis limbs, it is safe to assume that the anterior pair, 

 corresponding to the antennules, must be uniramous, since 

 they are so during all the young and adult stages observed, 

 and since this form is common to all nauplius stages of modern 

 Crustacea, and is recognized as primitive and elementary for 

 the class. There is apparently a greater similarity in the 

 larval antennules than between any other appendages, and as 

 Apus and Euphausia have these in a very generalized form, 

 they are taken as types of the first pair of limbs of the trilo- 

 bite protaspis, as shown in Plate V, figure 1 (/). It siiould 

 be noted, too, that the antennules of the trilobites arise from 

 the sides of the upper lip or hypostoma, as in the nauplius. 



The other head appendages are typically branched, though 

 in many of the recent Crustacea they lose tliis character after 

 the larval stages. Especially is this true of the third pair of 

 limbs, which become modified into the mandibles. In trilo- 

 bites the primitive biramous structure of the head limbs per- 

 sists to adult stages, occurring also in limbs of all the posterior 

 segments where they become more and more phyllopodiform.^ 

 In the restoration of the protaspis it seems only necessary to 

 append this archaic type of limb to each segment, agreeing as 

 it does in form and structure with the rudimentary limbs of 



