SOMITES AND APPENDAGES. 143 
fourth and fifth, and the fifth and sixth segments, the fifth 
will be isolated, and can be studied apart. It constitutes 
what is called a metamere ; in which are distinguishable a 
central part termed the somite, and two appendages 
(fig. 36). 
In the exoskeleton of the somites of the abdomen 
several regions have already been distinguished; and 
although they constitute one continuous whole, it will 
be convenient to speak of the sternum (fig. 36, st. XIX), 
the tergum (¢. XIX), and, the pleura (pl. XIX), as if they 
were separate parts, and to distinguish that portion of 
the sternal region, which lies between the articulation 
of the appendage and the pleuron, on each side, as the 
epimeron (ep. XIX). Adopting this nomenclature, it may 
be said of the fifth somite of the abdomen, that it 
consists of a segment of the exoskeleton, divisible into 
tergum, pleura, epimera, and sternum, with which two 
appendages are articulated; that it contains a double 
ganglion (gn. 12), a section of the flexor (fm) and extensor 
(em) muscles, and of the alimentary (hg) and vascular 
(s.a.a, t.a.a) systems. 
The appendage (fig. 36, 19), which is attached to an 
articular cavity situated between the sternum and the 
epimeron, is seen to consist of a stalk or stem, which is 
made up of a very short basal joint, the coxopodite (fig. 37, 
D and EH, cx.p), followed by a long cylindrical second 
joint, the basipodite (b.p), and receives the name of pro- 
lopodite. At its free end, it bears two flattened narrow 
