210 STUDIES IN EVOLUTION 



Observations. 



With these additional discoveries relating to Triarthrus, 

 several observations upon its general organization and com- 

 parisons with other Crustacea may be made. This cannot be 

 done exhaustively or comprehensively at this time, and only 

 a few points will be touched upon. The simplicity and 

 primitiveness of the trilobite structure will first impress the 

 student. The variable number of segments in the thorax 

 and pygidium in the different genera shows the unstable 

 metameric condition of the class. The head alone seems to 

 have a permanent number of segments and appendages. 

 Even this is not often apparent, but the constant number 

 of five head segments in larval trilobites shows this to be the 

 true number, although subsequent growth may obscure or 

 obliterate this pentasomitic character, as has been shown by 

 the writer in Acidaspis (Amer. Jour. Sci., August, 1893) 

 and observed in other genera. 



With the exception of the antennules, all other paired 

 appendages of the animal seem to agree in every point of 

 structure, and vary only in the relative development of cer- 

 tain parts. The appendages of the pygidium are ontogeneti- 

 cally the youngest, and express the typical phyllopodiform 

 structure. Passing anteriorly, the joints become less leaf- 

 like, until in the anterior thoracic legs they are quite slender, 

 and the limbs resemble those of schizopods. Corresponding 

 to this, there is, through the whole series, a gradual develop- 

 ment of a process from the coxopodite, forming a gnathobase 

 to the limb. On the head these serve as true manducatory 

 organs. Posteriorly they were like the basal endites of 

 Apus, and enabled the trilobite to convey food along the 

 entire length of the axis to the mouth. 



Bernard (I. c.) has made a strong exposition of the evi- 

 dence in favor of the phyllopod affinities of the Trilobita, 

 and especially of their relations to Apus. A portion of the 

 under side of the head of Apus is introduced for comparison 

 on Plate VIII, figure 12. Both pairs of antennal organs 



