CRUSTACEA. 109 



3. Myriapoda — Centipedes and Millipedes. Head quite separate fium tlie 



body, bub no distinction between thorax and abdomen visible. One 

 pair of antenn;c. Abdomen composed of many segments. Legs very 

 numerous, always more than eight pairs. Tracheal respiration. 



4. Hexapoda — Head, thorax, and abdomen distinct. One pair of antenniE. 



Eyes both simple and compound. Legs always six in number. No 

 appendages of locomotion on the abdomen. Two pairs of wings 

 generally present. Respiration tracheal. 



Crustacea. 



The Crustacea are chiefly aquatic arthropods. Some few 

 species live in damp earth and in damp general surroundings 

 ("Wood-lice and Crawfish). The Wood-lice belong to the order 

 Isopoda . they may be told by the equal and symmetrical feet 

 or legs, whilst in the others, such as the Crab and Lobster, the 

 claws are asymmetrical. There are three species injurious to 

 horticulturists in Europe, all of which 

 belong to the family Oniscidw. The 

 Oniscidas belong to the second tribe of 

 Isopoda, called the Euisopoda, which 

 have the body made up of seven free 

 thoracic segments and the same number 

 of paired appendages. The abdominal |S^ h5v\ 

 feet are so modified as to form branchial ^m^ ^^^^^ ^<3=»*' 

 lamellae. The Onisoids or "Wood-lice ^ ^^"^ 



(fig. 44) are land Isopods, with the Fic. 44.-Wood-lice. 



endopodites of the abdominal feet Potc^SSn o, l-'lSf^aii"; 

 formed into branchiaj, the exopodites ?^nna'rf pV/eiuo.""" ' "' ™" 

 into protective lateral plates on the 



outside. Peculiar modifications may be seen on the two front 

 abdominal feet, which are provided with special sacs to contain 

 air. The three common species are Oniscus asellus of Linnaeus, 

 Porcellio seaber of Latreille, and Armadillidium vulgaris of 

 Linnaeus (fig. 44, a). They are popularly called Monkey-peas or 

 Slaters in Britain, the Cow-bugs of America ; perhaps the term 

 Wood-louse is that most usually employed. 



