491 



CHAPTER XXVIII. 



ARTHROPODA. 



Crustacea. 



The division of the Arthropoda or " Articulate Animals " com- 

 prises the four great classes of the Crustacea (Lobsters, Crabs, &c), 

 the Arachnida (Spiders and Scorpions), the Myriopoda (Centipedes 

 and Millepedes), and the fnsecta (the Insects properly so called), 

 and is distinguished as follows : — 



The body is composed of a series of segments or "somites" zvhich 

 are usually definite in number, and which are arranged along a lon- 

 gitudinal axis. Each segment may be provided ivith a single pair of 

 appendages, and these are always jointed, and are articulated to the 

 body. Both the segmented body and the articulated appendages are, 

 as a rule, furnished with a chitinous, often more or less extensively 

 calcified, exoskeleton, formed by a hardening of the integument. The 

 appendages are hollow, and the muscles are prolonged into their in- 

 terior. The nervous system consists, typically, of a double chain of 

 ganglia placed along the ventral surface of the body, united by longi- 

 tudinal commissures, and traversed anteriorly by the oesophagus. The 

 heart, when present, is situated dorsally. When distinct respiratory 

 organs are prese?it, they may be in the form of gills or branchice, or 

 they may be saccular or tubular involutions of the integument (pul- 

 monary sacs or tracheal) adapted for breathing air directly. 



If the King-crabs and the Eurypterids be retained in the Crus- 

 tacea, then the classes of the Arachnida, Myriopoda, and Insecta 

 comprise those Arthropods which breathe air directly, and which 

 are thus, in general, adapted for a terrestrial life. On the other 

 hand, the Crustacea are essentially water-breathing animals, and 

 even those forms which are terrestrial in their habit, possess respir- 

 atory organs in the form of branchiae. From their generally aquatic 

 mode of life, and the usually more resistant character of their exo- 

 skeleton, the Crustacea are more abundantly represented in the 



