STRUCTURE AND CLASSIFICATION OF THE ARTHROPODA. 559 



It may be pointed out that the most radical difference 

 presented in this list is that between appendages consisting 

 of the corm alone without rami (Onychophora) and those 

 with more or less developed rami (the rest of the Arthropoda). 

 In the latter class we should distinguish three phases: (a) 

 those witii numerous and comparatively undeveloped rami; 

 (b) those with tln-eCj or two highly developed rami, or with 

 only one — the corm being reduced to the dimensions of a 

 mere basal segment; (c) those reduced to a secondary 

 simplicity (degeneration) by overwhelming development of 

 one segment (e. g. the isolated gnathobase often seen as 

 "mandible" and the genital operculum). 



There is no reason to suppose that any of the forms of 

 limb observed in Arthropoda may not have been indepen- 

 dently developed in two or more separate diverging lines of 

 descent. 



Branchias. — In connection with the discussion of the limbs 

 of Arthropods a few words should be devoted to the gill- 

 processes. It seems probable that there are branchial plumes 

 or filaments in some Arthropoda (some Crustacea) which can 

 be identified with the distinct branchial organs of Chsetopoda, 

 which lie dorsad of the parapodia and are not part of the 

 parapodium. On the other hand, we cannot refuse to admit 

 that any of the processes of an Arthropod parapodium may 

 become modified as branchial organs, and that, as a rule, 

 branchial outgrowths are easily developed, de novo, in all 

 the higher groups of animals. Therefore it seems to be, 

 with our present knowledge, a hopeless task to analyse the 

 branchial organs of Arthropoda and to identify them genetic- 

 ally in groups. 



A brief notice must suffice of the structure and history of 

 the Kyes, the Trachete, and the so-calJed Malpighian 

 tubes of Arthropoda, though special importance attaches 

 to each in regard to the determination of the affinities of the 

 various animals included in this great sub-phylum. 



The Eyes. — The Arthropod eye appears to be an organ of 

 special character developed in the common ancestor of the 



