No. 625] THE EVOLUTION OF ARTHROPODS 151 



usually very greatly modified in adaptation to its own 

 mode of life and environmental conditions, and fre- 

 quently represents an interpolated stage having no great 

 . phylogenetic significance (in comparison with the devel- 

 opmental stages of the embryo). Furthermore, it is 

 extremely probable that the ancestral arthropods were 

 not of one single type at all, but doubtless differed very 

 greatly among themselves in size, the number of seg- 

 ments composing their bodies, etc., just as is the case 

 among certain annelidan groups, or as is the case among 

 the assemblage of lower arthropods comprising the most 

 primitive members of the group next to be discussed. 



The assemblage of lower arthropods comprising the 

 Copepoda, Branchiopoda and their immediate relatives 

 may be referred to as the Copepoda-Branchiopoda group. 

 Its members include some of the most primitive of the 

 arthropods, and it may be regarded as representing as 

 nearly as any the forms giving rise to the different 

 arthropodan lines of develo]^in('iit. The Ostracoda rep- 

 resent a line of development wliicli branclicd off at an 

 early date, and should also be 'mchuled in the group; but 

 they are not structurally so important as the Branchio- 

 poda, etc., for a phylogenetic study of the lines of descent 

 to which the ancestral arthropoda gave rise. The Cir- 

 repedia likewise reju'csont a grou]) wliicli 1)raiK'lied off 

 from this stem nt an (s-irly date, hut tlicy aiv to,. <1, -gen- 

 erate, and ]iav(- f.^llowed tlirir ..wn line ..f ^lu'riali/.ation 

 too far to be iiicliKled aiiioug the ])riinitive ivi.rc-e;ita- 

 tives of the Copepoda-Branchiopoda groiij). The Trilo- 

 bita are very closely related to the Apodia> and P)ranc]iio- 

 poda in general, for such trilobites as Xatlmrstln Iransi^ 

 ians are soniewliat annectent betwcn-n the ti-ih.hitcs and 

 the branelii. .{.(..] Opnlnun n-nolis dt-crihtHl l.y Wah-.ttt, 

 19lL'. an<l sn.-li irih. hires as MarJIn splrwlrns aiv very 

 like certain Apodida'. ('t<-.; l>nt the r\o-v>\ al'linilic^ of the 

 Trilobita ai-pcar t(. lie with tlie group iirxl lo he ron-id- 

 ered, and altluHigli th." trih)hilr> have piv-rrvcd many 

 very primitive iVatiuTs which might eiitith- them to a 



