Sub-Kingdom VII. ARTHROPODA. (Arricunarss.) 
Heteronomously segmented animals with, typically, a pair of appendages to each 
somite of the body ; the whole enclosed in a chitinous segmented exoskeleton, the joint- 
ing of which extends to the appendages. 
In the Arthropoda the segments are unequally developed, and_ the 
appendages, primitively locomotor in function, may be modified on one or 
more somites to subserve special functions, such as the seizure and comminu- 
tion of food, respiration, sensation, copulation, oviposition, fixation, ete. 
These modifications of the appendages and the more or less complete union 
of the segments into groups may result in the differentiation of three distinet 
regions: head, thorax, and abdomen. Of these regions the head is concerned 
largely in sensation and feeding, the thorax is chiefly locomotor in function, 
and the abdomen frequently defensive. 
The brain lies above and in front of the oesophagus, and consists of a 
fusion of several pairs of ganglia. ‘The rest of the central nervous system con- 
sists of a chain of ganglia lying in pairs on the ventral surface, with typically 
a pair in each somite. Not infrequently there is a more or less extensive con- 
centration or fusion of these ventral ganglia. The eyes may be simple, 
aggregate, or compound, with in some cases an inversion of the retinal layer. 
Respiration in the smaller forms is by the general surface of the body, 
whereas in the larger certain regions become specialised for this purpose. 
When respiratory outgrowths protrude from the body wall they are known 
as gills or branchiae; when invaginated they are termed Jungs if they be 
lamellar in arrangement, or tracheae if they consist of fine tubes ramifying 
through the tissues. 
Excretion is effected either by “segmental organs” (true nephridia) 
which open at the inner end into the true body cavity (coelom) and at the 
other to the exterior, or by diverticula developed at the hinder end of the 
alimentary canal. The nephridia when present occur in only a few segments 
of the body. The diverticula of the alimentary canal (Malpighian tubes) are of 
two kinds—one developed from the mesenteron, the other from the procto- 
derm. In all Arthropods the ducts of the reproductive organs are appar- 
ently modified nephridia, and the organs themselves consist of gonads 
developed from the coelomic walls. The circulation depends upon a dorsal 
heart enclosed in a vascular pericardial sac, and metameric blood-vessels 
terminating in “lacunar” spaces. 
Arthropods are divisible into three groups or sub-phyla, distinguished 
according to the nature of the respiratory organs, segmentation of the body, 
