No. C25] THE EVOLUTION OF ARTHROPODS 155 



that the fossil annelids, Canadia spinosa (in which the 

 head is bent down "so that the mouth faces posteriorly" 

 in the position assumed by Bernard, 1892, to be that of 

 the annelids which gradually took on the character of 

 head region leading up to the arthropod type), and the 

 Crustacea "were derived from the same general type of 

 animal." The Copepoda represent a line of develop- 

 ment which branched off near that of the Branchiopoda, 

 at the base of the arthropod stem; and the Argulidge 

 (which are grouped with the Coi^epoda by Caiman, 1909) 

 are regarded by some authorities as annectent between 

 the Copepoda and tlic l>raitclii()]ioda. The Ostracoda 

 are related to both the coiicliostiacan and cladoceran 

 Branchiopoda (following Caiman's classification) and 

 the ancestors of the ostracods doubtless arox- t'l .tin l'(»i ins 

 intermediate between the Cladocera and (On.lio-traca. 

 The Cirrepedia are apparently descended I'ldni nii.-c-tors 

 related to both the Ostracoda and CoiH'pn.la. and llieir 

 line of development branched off at ;iti carlx date to fol- 

 low their own strongly aberrant i)art of (U'xclopnicnt. 



Such anostracan l)rancliiopod> a< tiio fo->il Opahina 

 regalis, whose structnrc accoi-dinu' to \\';d<-ott, 1912, "is 

 very suggestive of an anndiihin ancestor." and such 

 notostracan branchiopods as the fo--il Hnif/cssia hella 

 (which has sessile eyes and liei»atic uhind^ in a carapace 

 resembling that of Lcpt'ln nts) sei ve to indicate wlint the 

 first arthropods were ].i-ol)ably like, and they dcmi^y a 

 position near the base of tlio steni-fornis who-e lines of 

 development were eventually to prodnce tlie in^ectan 

 type of arthropod. The fossil noto-trnran l.ranchiopod 

 Waptia occupies a position annectent Ix iween the above- 

 mentioned branchioiK)ds and the malnco^tracan uionp 

 next to be considered. 



The leptostracan ().hvlh.cai'id ) u'l^Mip oc.-npio a po- 

 sition intermediate l^etweeii the re<t of thi^ Malaco-t raca 

 and the branchiopods described aho\e. They have idso 

 carried over from their coininon hranchiopod ance>lry 

 certain features likewise inherited by the trilol)ites; but. 



