ANNULOSA: CRUSTACEA. 279 



appendages, is enclosed in a strong chitinous " shell," or exo- 

 skeleton, and the cephalothorax is covered by a great cephalic 

 shield or plate, which is termed the " carapace." 



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. 137, 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 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 

 "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. 135). 



It must be borne in mind that though the so-called "head" that is to 

 say, the "cephalothorax" of the Lobster is produced by an amalgama- 

 tion of the various somites of the head and thorax, this is not the case with 

 the great shield which covers this portion of the body. This shield the 

 so-called "cephalic buckler," or "carapace" is not produced by the 

 union of the tergal arcs of the various cephalic and thoracic segments, as 

 would at first sight appear to be the case. On the contrary, the " cara- 

 pace " in the higher Crustacea is produced by an enormous development 

 of the tergal pieces, or of the " epimera" or one or two of the cephalic 

 segments : the tergal arcs of the remaining somites being overlapped by 

 the carapace and remaining undeveloped. 



Examining the somites from behind forwards (for simpli- 

 city's sake), the last segment comes to be first described. This 

 is the so-called "telson," which forms the last articulation 

 of the abdomen, and never bears any appendages. For this 

 reason, many authorities do not regard it as a somite, properly 

 speaking, but simply as an azygous appendage that is to say, 

 as an appendage without a fellow. In the next segment (the 

 last but one, or the last, of the abdomen, according to the view 

 which is taken of the " telson "), there is a pair of natatory ap- 

 pendages, called " swimmerets." Each swimmeret (fig. 137, 2) 

 consists of a basal joint, which articulates with the sternum, 

 and is called the " protopodite " or propodite, and of two 

 diverging joints, which are attached to the former; the outer 



