CLASSIFICATION 5 



pidae). Stigmata paired and segmentally arranged. A meta- 

 morphosis (direct or indirect) occurs except in Thysanura and 

 Collembola. 



Relationships. The interrelationships of the classes of 

 Arthropoda form an obscure and highly debatable subject. 



Crustacea and Insecta agree in so many morphological 

 details that their resemblances can no longer be dismissed as 

 results of a vague " parallelism," or " convergence " of devel- 

 opment, but are inexplicable except in terms of community of 

 origin, as Carpenter has lately insisted. 



Arachnida are extremely unlike other arthropods but find 

 their nearest allies among Crustacea, particularly the fossil 

 forms known as trilobites. 



Malacopoda, as represented by Peripatus, are often spoken 

 of as bridging the gulf that separates Insecta, Chilopoda and 

 Diplopoda from Annelida. Peripatus indeed resembles the 

 chaetopod annelids in its segmentally arranged nephridia, 

 dermo-muscular tube, coxal glands and soft integument, and 

 resembles the three other classes in its tracheae, dorsal vessel, 

 lacunar circulation, mouth parts and salivary glands. These 

 resemblances, however, are by no means close, and Peripatus 

 does not form a direct link between the other tracheate arthro- 

 pods and the annelid stock, but is best regarded 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 themselves have been de- 

 rived. 



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 characters 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 Insecta than 

 are Diplopoda, as regards segmentation, mouth parts, tracheae, 

 genital openings and other characters. 



