464 



ARTICULATES : CRUSTACEANS. 



SUB-SECTION II. 



THE ORDER OF TETRADECAPODS, OR FOURTEEN-FOOTED CRUS- 

 TACEANS. 



THE Order of Tetradecapods comprises the Crustacea 

 which normally have seven cephalic segments and pairs 

 of appendages, and seven foot-rings or pairs of feet. It 

 contains three groups, - - Isopods, including Anisopods ; 

 Amphipods, including Laemodipods ; and Trilobites. The 

 living species are mainly very small. 



The Isopods have the four posterior pairs of thoracic 

 legs in one series, and the three anterior in another ; the 

 branchiae abdominal ; and the abdominal members in 

 two sets, the five anterior pairs branchial, and the sixth 

 more or less styliform. Many of the marine species are 

 parasitic on other animals. The land genera inhabit 

 dark, damp situations. Of the latter, Oniscns, containing 

 the Sow-Bug, and Armadillo, .the Pill-Bug, are examples. 

 The Amphipods have the three poste- 

 rior pairs of thoracic legs in one series, the 

 four anterior pairs in two other series of 

 two pairs each, and the branchiae thoracic ; 

 the abdominal members in two sets, the 

 three anterior pairs subnatatory, the three 

 posterior styliform. They are known as 

 Sand- and Beach-Fleas. 



Trilobites may perhaps 

 be placed here, according 

 to the classification pre- 

 sented by Dana in his learn- 

 ed papers on Cephaliza- 

 tion. These very curious 

 animals are all fossil, and 

 abound in the Silurian and 

 Devonian rocks. They also 

 occur in the Carboniferous, where they at last disappear. 



Fig. 364. 



Sand-Flea, 



Orchestia longicor- 



tn's, Gould. 



Fig. 3 6 5- 



