( I VSSI1 l VI 



Crustacea and in ecta a<:rer tn so many morpholog ' hat 



their resemblances can no longer l>< - of a \ 



"parallelism," or "convergence " of de\ elopment , bir 

 except in terms of community of ipmtrr 



Arachnida are extremely unlike other arthropods hut find their 

 Dearest allies amon.^ Crustacea. particu- 

 larly the fossil forms known as triln! 



Onychophora. as represented by I'cri- 

 pahis, are often spoken of as l>rid^in<^ the 

 ,milf that M-paratc-s Inseeta. Chilopoda and 

 Diploj)oda from Annelida. 1'cri pains in- 

 (K-ed resembles tlu> i-ha-topod annelids in 

 its scmnentally arranged nephridia, der- 

 niomuseular tube, coxal glands and soft 

 integument, and resembles the three other 

 i lasses in its tracheae, dorsal vessel with 

 paired ostia, lacunar circulation, mouth 

 parts and salivary glands. These resem- 

 blances are by no means close, however, 

 and Peri pat us does not form a direct link 

 between the other tracheate arthropods 

 and the annelid stock, but is best regard- 

 ed as an offshoot from the base of the 

 arthropodan stem. 



In speaking of annelid ancestors, none 

 of the recent annelids are meant, of course, 

 but reference is made to the primordial 

 stock from which recent annelids them- 

 selves have been derived. 



Though Diplopoda and Chilopoda have 

 long been grouped together under the name 

 Myriopoda, they really have so little in 

 common, beyond the numerous. limb-bearing segments and the charac- 

 ters that are possessed by all tracheate arthropods, that their differences 

 entitle them to rank as separate classes. Chilopoda as a whole are 

 more nearly related to Inseeta than are Diplopoda, as regards seg- 

 mentation, mouth parts, tracheae, genital openings and other characters. 



Scolopcndrclla, now placed in a class by itself, Symphyla, presents a 



rkable combination of diplopodan and insectean characters. 



Scolopcndrclla (Fig. 6) and the- thysanuran Campodca have the >ame 



FIG. 5. 



A centipede, Scoioptn- 

 tw - thirds the 



