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Subkingdom Y. ARTHROPODA. 



CONDYLOPODA. ARTICULATA. GNATHOPODA. 

 AETHEOZOA. 



Segmented, non-ciliated animals, with distinctly jointed legs. 

 Nervous system ganglionic. Sexes generally separate. Ovi- 

 parous. 



The Arthropoda, with more than 200,000 species, vary to such 

 an extent that little can be said applicable to the whole group. 

 Of all Invertebrata they are the most advanced in the develop- 

 ment of the organs peculiar to animal life, " manifested in the 

 powers of locomotion, and in the instincts, which are so varied 

 and wonderful in the insect class." 



The Arthropoda fall naturally into four classes, which A. 

 Agassiz, with cruel refinement, only ranks as orders. 



Head, thorax, and abdomen distinct INSECTA. 



Head or abdomen not distinct from the thorax. 

 With antennae. 



One pair MYRIOPODA. 



Two pairs CRUSTACEA. 



Without antennae ARACHNIDA. 



Class I. CRUSTACEA. 



AGON AT A. BRANCHIOPNOA. 



Mostly aquatic animals, with articulated appendages as well on 

 the thorax as on the abdomen? Two pairs of antennas in most. 



The segments of Crustacea consist, at least theoretically, of a 

 " tergum " of two pieces and an " epimeron " on each side above ; 

 and beneath of a "sternum," also of two pieces, and two lateral 

 " episterna." The hard processes of the internal skeleton are 

 the " apodemata," and serve, for the attachment of muscles &c. 



Some of the lower forms of Crustacea retrograde after passing 

 the embryonic stage, but an advancing and gradual metamor- 

 phosis is more general. Three larval forms may be distin- 

 guished : (1) " Nauplius" oval, unsegmented ; one eye; three 

 pairs of appendages, which are converted into antennae and 



