IQ4 MANUAL OF ZOOLOGY. 



in a strong chitinous " shell," or exoskeleton, and the cephalo- 

 thorax 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 out- 

 wards, into a plate, which is called the " pleuron," or "pleura" 

 ( fi g- 59, 2 )- 



Strictly speaking, the composition of the typical somite is considerably 



more complex, each of the primary arcs of the somite being really composed 



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



each side of the middle line of the body, 



t united together, and constituting the "ter- 



e e gum " proper. The superior arc is com- 



W^ D ^^ pleted by two lateral pieces, one on each 



p '""^.. "M"'^ Sl de of the tergum, which are termed the 



l^k * jjr "epimera." In like manner, the ventral 



f j? or sternal arc is composed of a central plate, 



s s composed of two pieces united together in 



Fig. 5 8.-Theoretical figure illus- the iddle line > and constituting the ster- 



trating the composition of the num proper ; the arc being completed by 



tegumentary skeleton of the t wo lateral pieces, termed the " episterna." 



Crustacea (after Milne-Ed- These plates are usually more or less com- 



Tergal pieces ;T* Ephneral pletely anchylosed together, and the true 



pieces ; v. Ventral arc ; s JT structure of the somite in these cases is often 



Sternal pieces; //Episternal shown by what are called "apodemata." 



extremiui^ rtl n f ^ These <* se P ta which P roceed 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. 58). 



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 amalgamation 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 " carapace" in the higher Crustacea is produced by an enor- 

 mous development of the tergal pieces, or of the "epimera" 

 of one or two of the cephalic segments : the tergal arcs of the 

 remaining somites being overlapped by the carapace and re- 

 maining undeveloped. 



Examining the somites from behind forwards (for simplici- 

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

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



