174 MANUAL OF ZOOLOGY. 



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, united 

 . ,. together, and constituting the 'tergum' 



proper. The superior arc is completed by 

 p ^y? two lateral pieces, one on each side of the 



^ _.-fi tergum, which are termed the 'epimera.' 

 * In like manner, the ventral or sternal arc 

 is composed of a central plate, composed 

 ^T'^iT 3 f two pi eces united together in the 



middle line, and constituting the ' sternum ' 

 Fig. 48. Theoretical figure illus- proper; the arc being completed by two 



te a S m lnta?y C Tkeieton 0n of the lateral P ieces > termed the ' episterna,' These 

 Crustacea (after Milne-Edwards), plates are usually more or less completely 

 D. Dorsal arc ; 1 1. Tergal pieces; anchylosed together, and the true structure 



Epteternal pieces ; pp. Insertion by what are called 'apodemata.' These are 

 of the extremities. septa which proceed inwards from the in- 



ternal surface of the somite, penetrating 



more or less deeply between the various organs enclosed by the ring, and 

 alwavs proceeding from the line of junction of the different pieces of the 

 segment (Jig. 48). 



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 to 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 re- 

 maining somites being over- lapped by the carapace and remain- 

 ing undeveloped 



Examining the somites from behind forwards (for sim- 

 plicity'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 azygos 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- 

 appendages, called 'swimmerets.' Each swimmeret (fig. 49, 2) 

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

 and is called the ' protopodite ' or propodite, and of two di- 

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

 of these being called the ' exopodite,' and the inner the * endo- 



