Phylum VII. ARTHROPODA. (Articulates.) 



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 aad comminu- 

 tion of food, respiration, sensation, copulation, oviposition, fixation, etc. 

 These modifications of the appendages and the more or less complete union 

 of the segments into groups may result in the differentiation of three distinct 

 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 braii| 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 S2:)ecialised for this purpose. 

 When respiratory outgrowths protrude from the body wall they are known 

 as gills or branchiae ; when invaginated they are termed Inngs 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- 

 daeum. In all Arthropods the ducts of the reproductive organs are apparently 

 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 subphyla, distinguished 

 according to the nature of the respiratory organs, segmentation of the body, 



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