CLASSIFICATION 



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 development, but are inexplicable 

 except in terms of community of origin, as Carpenter has insisted. 



Arachnida are extremely unlike other arthropods but find their 

 nearest allies among Crustacea, particu- 

 larly the fossil forms known as trilobites. 



Onychophora, as represented by Peri- 

 patus, are often spoken of as bridging the 

 gulf that separates Insecta, Chilopoda and 

 Diplopoda from Annelida. Peripatus in- 

 deed resembles the chaetopod annelids in 

 its segmentally arranged nephridia, der- 

 momuscular tube, coxal glands and soft 

 integument, and resembles the three other 

 classes 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 Peripatus 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 Insecta than are Diplopoda, as regards seg- 

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



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

 remarkable combination of diplopodan and insectean characters. 

 Scolopendrella (Fig. 6) and the thysanuran Campodea have the same 



PIG. 5. A centipede, Scoiopen- 

 **** the 



