530 Causes and Course of Organic Evolution 



fers Pedalion has by them been regarded as an examjile of 

 parallel or convergent development between groups that are 

 distinct. Such a view is entirely tenable, if no more helpful 

 one can be advanced. We would however decidedly favor 

 the view that the many structural and developmental resem- 

 blances shown by some rotifers and specially Pedalion to primi- 

 tive and mainly fresh-water entomostracan genera, as well 

 as to larval stages of higher arthropods, indicate a real and 

 close phylogenetic affinity. So, instead of tracing the iVrthro- 

 l)oda from an annelidan ancestry, we would derive it directly 

 from the Rotifera. 



The early larval or nauplius body of Crustacea shows — 

 like that of all previous groups which we have regarded as 

 descended from the Rotifera — division into a preoral or cephalic 

 region, a body or postoral or thoracic region, and a tail or 

 caudal or abdominal region. Though it is still difficult to 

 correlate the exact organs or appendages that each of these 

 may bear, every new investigation is adding to our ability 

 in effecting this. Such a division as that now given is fully 

 recognized by Korschelt-Heider {17 2 y II: 191, footnote) who 

 say: *'x\lthough there is no external segmentation in the nau- 

 plius body, we can nevertheless recognize in it the following 

 segments: an anterior preoral or primary cephalic segment, 

 a posterior terminal or anal segment at the posterior end of 

 the body, and the true trunk segments which lie between these." 



A striking difference in the surface aspect of the crustacean 

 larva, as compared with those already treated of, is the absence 

 of cilia or ciliated areas, so that one has largely to be guided 

 by the average position and historical relation of parts in corre- 

 lation efforts. Reasons for this ciliary absorption seem to 

 be, increased thickness of the egg wall, retention of the young 

 larva within it till it has formed swimming organs, precocious 

 formation and functioning of the latter, and abundant storage 

 of rich yolk food in many cases — Entomostraca excepted — 

 that enables precocious larval development to be effected. 



In w^hat follows we would premise that the Crustacea of 

 entomostracan affinity are the most primitive arthropods, 

 from which by increased segmentation and specialization, most 

 of the other arthroi)od groups are derived. 



Probably the most fundamental point that might first be 

 considered is the origin and value of the paired, jointed seta- 

 ceous limbs that are so characteristic of nearly all larval crus- 



