ANNUJLOSA: ARTHROPODA. 275 



2. ARACHNIDA. Respiration by pulmonary vesicles, by trachea, 

 or by the general surface of the body. Head and thorax united 

 into a cephalothorax. Antenna (as such) absent. Legs eight. 

 Abdomen without articulated appendages. 



3. MYRIAPODA. Respiration by trachea. Head distinct ; re- 

 mainder of the body composed of nearly similar somites. One pair 

 of antenna. Legs numerous. 



4. INSECTA. Respiration by trachea. Head, thorax, and 

 abdomen distinct. One pair of antenna. Three pairs of legs 

 borne on the thorax. Abdomen destitute of limbs. Generally 

 two pairs of wings on the thorax. 



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CHAPTER XXX. 



CRUSTACEA. 



CLASS I. CRUSTACEA. The members of this class are com- 

 monly known as Crabs, Lobsters, Shrimps, King-crabs, Bar- 

 nacles, Acorn-shells, &c. They are nearly allied to the suc- 

 ceeding order of the Arachnida (Spiders and Scorpions) ; but 

 may usually be distinguished by the possession of articulated 

 appendages upon the abdominal segments, by the possession 

 of two pairs of antennas, and by the presence of branchiae. 



The body is composed of a number of definite rings or seg- 

 ments (" somites "), each of which may be provided with a pair 

 of jointed appendages. With rare exceptions, some of the 

 somites of the adult always carry appendages ; and one or 

 more pairs are almost invariably adapted for mastication. The 

 nervous system of the embryo has the typical Annulose form 

 of a chain of ventral ganglia, between the first two pairs of 

 which the gullet passes. No water-vascular system is present ; 

 but there is generally a true blood-vascular system. The heart, 

 when present, is placed on the opposite side of the alimentary 

 canal to the ventral nerve-chain, and communicates by valvular 

 apertures with a surrounding venous sinus the so-called " peri- 

 cardium." When differentiated breathing-organs are present, 

 these are always in the form of branchiae or gills, adapted for 

 respiring air dissolved in water. 



In addition to these characters, the body in the Crustacea 

 is always protected by a chitinous or sub-calcareous exoskele- 

 ton, or " crust," and the number of pairs of articulated limbs 

 is generally from five to seven. They all pass through a series 



