ENTOMOLOGY 



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 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, particularly the fossil 

 forms known as trilobites. 



Malacopoda, as represented by Perip- 

 atus, 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, dermo- 

 muscular tube, coxal glands and soft integu- 

 ment, and resembles the three other classes 

 in its tracheae, dorsal vessel, lacunar circu- 

 lation, 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 

 arthropods 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 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 characters that are possessed by all tra- 

 cheate 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. 



Scolopendrella, now placed either among Diplopoda or else in a class 



FIG. 5. A centipede, Scolo- 

 pendra heros. About two-thirds 

 the maximum length. 



