INTRODUCTION. XV 



is yet susceptible of being reduced to a perfect theoretical 

 idea. Indeed, in many forms, the parts of which each 

 segment is composed are distinctly appreciable by careful 

 examination ; and it is found that these parts consist in 

 two arches, a superior and an inferior, each of which is 

 formed of two middle and two la- 

 teral pieces. The superior central 

 pair, a a, constitute the tergum, 

 the lateral are called epimera, b b. 

 Of the inferior arch, the two central pieces form the ster- 

 num, c <?, and to the lateral, d d, the name of episternum 

 has been applied. As we have already seen, in enume- 

 rating the segments themselves which compose the different 

 regions of the body, that some or other of them are always 

 found to be so intimately combined together that their dis- 

 tinction is lost, so in the present case also, some or other 

 of the theoretical elements of the segments are either actu- 

 ally wanting, or certain of them are so intimately united 

 that the normal number cannot be distinguished. 



It is also necessary, in order to obtain a correct idea of 

 the actual structure of the skeleton or supporting organs 

 in the Crustacea, to consider those processes of crustaceous 

 matter which, in the form of internal lamina, form the 

 parietes of the cells and canals which are found in the 

 interior of these animals, and many of which serve the 

 office of bones, as the solid surfaces to which the muscles 

 are attached. These have received the name of apodema, 

 " They arise in all cases from the junction of two con- 

 tiguous pieces of one segment, or from the union of two 

 rings. They are produced by a duplicature of the tegu- 

 mentary membrane, which dips more or less deeply amongst 

 the internal organs, and which becomes encrusted with 

 calcareous matter with the rest of the shell ; they are con- 



