CLASSIFICATION 5 
pide). 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 
chetopod annelids in its segmentally arranged nephridia, 
dermo-muscular tube, coxal glands and soft integument, and 
resembles the three other classes in its tracheze, 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, trachee, 
genital openings and other characters. 
