146 ANNULOSA. 



really consists of fourteen coalescent segments. The cephalo- 

 thorax is generally covered by a great shield or buckler, which 

 ( is termed the "carapace" (fig. 94, i, ca), which is produced 

 by an enormous development of the dorsal walls of one or two 

 of the cephalic somites. 



Each segment of the body may be regarded as essentially 

 composed of a convex upper plate, termed the " tergum," 

 which is closed below by a flatter plate, called the " sternum," 

 the line where the two unite being produced downwards and 

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

 "pleura" (fig. 94, 2). 



Strictly speaking, the composition of the typical somite is considerably 

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

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

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

 stituting the " tergum " proper. The superior arc is completed by two 

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

 "epj.mera." 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. 95). 



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 

 different functions. Usually, how- 

 e ever, a common morphological type 

 ^?.... p may be recognised in the appe'nd- 

 V ji^ ages of the C/7/j7Vta?tf, though certain 



f elements of this type are often want- 

 s s ing or much modified. Typically, 



Fig. 95. -Theoretical figure iiius- the appendages of the Crustacea 



tratmg the composition of the tegu- . rr _ 



mentary skeleton of the Crustacea COnSlSt Of an Undivided basal por- 



(after Milne-Edwards). D Dorsal f ' " r>rrrtrHi'f^ " m'xrinn- rn-i'm"r 



arc; /^Tergal pieces; ee Epime- tlOn r .Prpppaite, ^ giving Origin 



ral pieces ; V Ventral arc ; s * Ster- tO tWO diverging lOUltS. of which 



nal pieces ; ff Episternal pieces : , -, ' fi i . i / , T 



//insertion of the extremities. the inner is called the "endopo- 



dite," whilst the outer is known 



as the " exopodite." In such an appendage as the " swim- 

 meret " oif a Lobster (fig. 94, 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. 



