SUB-KINGDOM II.— ARTHROPODA. 



CRUSTACEA, INSECT A, &c. 

 By W. F. Kirby, F.L.S., F.E.S., &c. 



The Jrt/iropodd (otherwise na\\e^ Articnlata ov Annul osa) include an enor- 

 mous number of animals with an external skeleton, or, at least, a thickened 

 integument to which the muscles are attached ; and a body divided into 

 segments, and furnished with jointed organs for locomotion, etc. Many of 

 them undergo a metamorphosis, passing through several dissimilar stages 

 before reaching the perfect state in which they become sexually mature, and 

 capable of reproducing their kind. The sexes are generally distinct. They 

 breathe by trachefe, or by gills, and the nervous system consists of a series 

 of ganglia, of which the largest, situated in the head, are considered to 

 correspond to the brain. The blood is usually white. 



The following classes may be grouped under this heading : — 



I. Crustacea (Crabs, Lobsters, Shrimps, etc.). 

 n. Arachnida (Spiders, Scorpions and Mites). Classes of 



III. Myriopoda (Centipedes and Millepedes). ArtHropoda. 



IV. Protrachbata. 

 V. Insbcta (Insects). 



The characters of these classes are noticed in their places, and to enumer- 

 ate them here would be repetition. It must not be forgotten that all organic 

 beings show complicated affinities all round, and that no linear arrangement 

 can possibly be natural, and, therefore, we can only follow what appears to 

 be a fairly natural sequence, always remembering that by laying stress on 

 some characters we are necessarily ignoring others of perhaps equal or 

 greater importance ; and sometimes widely separating creatures which, when 

 looked at from a slightly diiferent point of view, are seen to be closely 

 allied. 



Space at disposal necessarily limits this article to a brief sketch of a vast 

 subject ; but care has been taken to preserve a certain proportion in allotting 

 the space to the different sections of which it is composed, and the informa- 

 tion given, though far from exhaustive, will be found to be useful and in- 

 structive. The selection made is that of the most important and interesting 

 groups into which these animals have been divided. These are dealt with in 

 the order given above, and are illustrated by numerous original drawings 

 reproduced in most cases life size. 



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