176 



THE AMERICAN NATURALIST 



[Vol.. LIII 



the Annelida, or even between the extremes included 

 within the annelidan group itself. 



The first arthropods also were not of one single type, 

 but possibly varied as greatly among themselves as a 

 branchiopod-like copepod would differ from a copepod- 

 like branchiopod, etc. It is very probable that the stem 

 forms eventually giving rise to the line of development 

 leading up to the production of the insectan type of 

 arthropod would be included in the branchiopod group. 



The next stage in the evolution of the insectan type of 

 arthropod is represented by forms related to the lepto- 

 stracan group, although the Leptostraca do not include 

 all of the types representing this stage of development. 

 It is possible that the Trilobita may be considered as 

 somewhat near these forms, since they exhibit a few char- 

 acters in common with them, but the trilobitan line of 

 descent is not directly in line with the insectan path of 

 development, since it diverges toward the evolution of 

 the merostomes and eurypterids leading off toward the 

 arachnoid type of development and away from the in- 

 sectan type. 



A further stage of developuKMil Tcjti-cx'ntod by tlie 

 members of the group includinu' the .\iioiii()>ti a('a. ( 'u- 

 macea and Tanaidacea. While tlic.x' doiilitless also re- 

 sembled the other members of this ur(»ii|t in certain re- 

 spects, it is quite possible that the aiiccstoi-s of insects 

 and "myriopods" varied between tlie extremes repre- 

 sented by Bathynella among the Anoniostraca and by 

 such forms as Apseudes, etc., among the Tanaidacea, 

 from which the Isopoda, etc., were also descended. 

 Bathynella, with no eyes, with its cylindrical body, re- 

 duced legs and "stumpy" pair of pleopods. basal limb 

 a])pen(la-es >nggesting the precursors of >tyli, short 

 tei-niiiiai a] 'pendau'es, etc., must be ver\- like the ances- 

 tors of til.' Pn.tnra and Scolopenrelloi.l fonns; while 

 such Ta.,ai,hi(..a n> .\,>sr,nJrs, xvith its flauvllitorm termi- 

 nal ni-o|iod>. an<l the t\])e of hea<l appeiKhiuvs, etc., pres- 



