ANATOMY OF AN ARTHROPOD. 89 



For this purpose we cannot choose a better example than the 

 Cockroach {Periplaneta americana), which can be easily ob- 

 tained, as our type. This species, although not so universal 

 as the common Cockroach, the so-called Black-Beetle of our 

 kitchens (Blatta orientalis), offers better scop6 for dissection, 

 being much larger. It may be obtained in numbers from any 

 of our docks, and is often present in such hordes on board ship 

 as to be a pest. 



The Cockroach. 



The whole insect will be observed at first sight to be covered 

 with a chitinous exoskeleton, which is thick and hard, dark 

 brown in colour, but paler and softer between the joints of the 

 somites. These joints divide the cockroach up into a number 

 of rings or segments, which are movable upon one another 

 except those of the head, these being firmly fused together. This 

 segmentation of the exoskeleton we shall observe marked inter- 

 nally, in the muscular and nervous systems. 



On examining the cockroach (fig. 31), which belongs to a 

 group of Insecta known as the Orthoptera, we shall at once 

 notice that the whole body can be divided into three parts — the 

 head [H), the thorax or chest region {Th), and the abdomen 

 {AV). The head is loosely connected with the thorax by a 

 narrow isthmus of tissue, the neck. The head is broad, and 

 bears jaws lying in a downward direction. The jaws will easily 

 be seen to work sideways or transversely, and not vertically as 

 ours do. Situated on the head are two large, black, reniform 

 elevations, the eyes {E), one on each side. These are the com- 

 pound eyes, each being composed of a number of hexagonal facets. 

 Situated at the inner side of the bases of the feelers or antennae 

 are two small white patches, the so-called fenestne (fig. 31, /, and 

 fig. 32 A, F). The antennae or jointed feelers will be seen to 

 spring from the base of the eyes in a depression on their anterior 

 edge. The dorsal and posterior surface of the head is called the 

 epicranium {Ep). Below the epicranium comes the elypeus (fig. 



