330 ARTHROPOD A. 



outwards into a plate which is called the " pleuroD," or 

 "pleura" (fig. 194, 2). 



Strictly speaking, the composition of the typical somite is considerably 

 more complex, each of the primary arcs of the somite being really com- 

 posed of four pieces. The tergal arc is composed of two central pieces, 

 one on each side of the middle line of the body, united together, and 

 constituting the " tergum " proper. The superior arc is completed by 

 two lateral pieces, one on each side of the tergum, which are termed the 

 " epimera." In like manner the ventral or sternal arc is composed of a 

 central plate, composed of two pieces united together in the middle line, 

 and constituting the "sternum" proper, the arc being completed by two 

 lateral pieces, termed the " episterna." These plates are usually more or 

 less completely anchylosed together, and the true structure of the somite 

 in these cases is often shown by what are called " apodemata." These 

 are septa which proceed inwards from the internal surface of the somite, 

 penetrating more or less deeply between the various organs enclosed by 

 the ring, and always proceeding from the line of junction of the different 

 pieces of the segment (fig. 195). 



Each somite of the body may bear a pair of appendages, 

 and these appendages are very much modified in different 



parts of the body, in order to fulfil 

 e different functions. Usually, how- 



w D ^^ ever, a common morphological type 

 P """<%. v W"~ ^ m &y be recognised in the appendages 



O f fa e Q rus t acea} though certain ele- 

 ments of this type are often wanting 



or much modmed - 



the tegumentary skeleton of a appendages of the Crustacea COllSlSt 



Crustacean (after Milne-Edwards). rr ' 



D, Dorsal are : t, t, Tergal pieces ; of an Undivided basal portion 01* 



c, e, Epiraeral pieces, v, Ventral ,, , . . . . 



arc :, s, Sternal pieces ; /,/, Epi- propodlte, giving Origin to two 



J oints > of wllich the inner 

 is called the " endopodite," whilst 



the outer is known as the " exopodite." In such an ap- 

 pendage as the "swimmeret" of a Lobster (fig. 194, 2), 

 these fundamental parts are readily recognisable ; but either 

 the exopodite or endopodite, or both, may be wanting, or 

 they may be very much modified in shape and form. 



It is impossible to give any general view of the appen- 

 dages of a Crustacean ; but it may be as well to name the 

 appendages which are present in one of the higher forms, 



