ANNULOSA : CRUSTACEA. 



CHAPTER XXXI. 



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 antennae, and by the presence of branchiae. 



The body is composed of a number of definite rings or 

 segments (" 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 "pericardium." 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 

 of metamorphoses before attaining their adult condition, and 

 ever} 7 part that is found in an embryonic form, even though 

 only temporarily developed, may be represented in a permanent 

 condition in some member of a lower order. , 



The classification of the Crustacea is extremely complicated, 

 and hardly any two writers adhere to the same arrangement. 

 The tabular view which follows embodies the arrangement 

 which appears to be most generally adopted, and the diagnostic 

 characters of each order will be briefly given, a more detailed 

 description being reserved for the more important divisions of 

 the class. Before proceeding further, however, it will be as well 

 to give a description of the morphology of a typical Crustacean, 

 selecting the lobster as being as good an example as any. 



