ANNULOSA: CRUSTACEA. 203 



wanting, and the antennae, though less liable to changes than 

 the remaining appendages, are nevertheless subject to very 

 extraordinary modifications, and have to perform functions 

 equally various. Being essentially and typically organs of 

 touch, hearing, and perhaps of smell, in the highest Decapods, 

 they become converted into burrowing organs in the Scyllaridce, 

 organs of prehension in iht Merostomata, c\.ss^^x% for the male 

 ill the Cydopoidea, and organs of attachment in the Cirripedia. 

 Not to multiply instances, we have presented to us in the 

 Crustacea probably the best zoological illustration of a class, 

 constructed on a common type, retaining its general character- 

 istics, but capable of endless modification of its parts, so as 

 to suit the extreme requirements of every separate species."^ — 

 (H. Woodward.) 



Taking the common Lobster (fig. 74) as a good and readily 

 obtainable type of the Crustacea, the body is at once seen to 

 be composed of two parts, familiarly called the " head " and 

 the " tail," the latter being jointed and flexible. The so-called 

 "head" is really composed of both the head, properly so 

 called, and the thorax, which have coalesced so as to form 

 a single mass, technically called the " cephalothorax." The 

 so-called "tail," on the other hand, is truly the "abdomen." 

 The various appendages of the animal are arranged along the 

 lower surface of the body, and consist of the feelers, jaws, 

 claws, legs, &c. The entire body, with the articulated ap- 

 pendages, 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 , e ^^ ,, 

 of a convex upper plate, termed the ^ 



" tergum," which is closed below by p .....T ^ 



a flatter plate, called the " sternum," ^^ ^ ^ 

 the line where the two unite being ' •'": '^^"'' / 

 produced downwards and outwards, ^ "' 



into a plate, which is called the '"l^^^^^Z'^^^S^^'S:, 



"pleuron," or "pleura" (fig. 62, 2). tegumentary skeleton of the 



^ ' ^ \ t) > / Crustacea (after Milne -Ed - 



Strictly speaking, the composition of the Xwgd piSeS"?"] Epiroeral 

 typical somite is considerably more complex, pieces; V Ventral arc; s s 



each of the primary arcs of the somite being Sternal pieces ; // Epistemal 



really composed of four pieces. The ter- Smiti4^ Insertion of the 



gal 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 



