STRUCTURE AND CLASSIFICATION OF THE ARTHROPODA. 527 



another so as to act as jaws. This is facilitated by an 

 important general change in the position of the parapodia ; 

 their basal attachments are all more ventral in position than 

 in the Ch^etopoda, and tend to approach from the two sides 

 towards the mid-ventral line. Very nsnally (but not in the 

 Onychophora = Peripatus) all the parapodia are plated 

 with chitin secreted by the epidermis, and divided into a 

 series of joints — giving the '^arthropodous'^ or hinged 

 character. 



There are other remarkable and distinctive featui-es of 

 structure which hold the Arthropoda together, and render it 

 impossible to conceive of them as haviug a polyphyletic 

 origin, — that is to sa}^, as having originated separately by two 

 or three distinct lines of descent from lower animals; aud, on 

 the contrary, establish the view that they have been deve- 

 loped from a single line of primitive Gnathopods which arose 

 by modification of parapodiate annulate worms not very 

 unlike some of the existing Chsetopods. These additional 

 features are the following : — (1) All existing Arthropoda 

 have an ostiate heart and have undergone "phleboedesis,'^ 

 that is to say, the peripheral portions of the blood-vascular 

 system are not fine tubes as they are in the Cha}topoda and 

 as they were in the hypothetical ancestors of Arthropod;), 

 but are swollen so as to obliterate to a large extent the 

 coelom, whilst the separate veins entering the dorsal vessel 

 or heart have coalesced, leaving valvate ostia (see Fig. l'^) by 

 which the blood passes from a pericardial blood-sinus formed 

 by the fused veins into the dorsal vessel or heart (see 

 Lankester's 'Zoology,' part ii, introductory chapter; A. 

 and C. Black, 1900). The only exception to this is in the 

 case of minute degenerate forms where the heart has dis- 

 appeared altogether. The rigidity of the integument caused 

 by the deposition of dense chitin upon it is intimately 

 connected with the physiological activity and form of all the 

 internal organs, and is undoubtedly correlated with the total 

 disappearance of the circular muscular layer of the body-wall 

 present in Cheetopods. (2) In all existing Arthropoda the 



