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 apijendages 

 (fig. 36). 



In the exoskeleton of the somites of the abdomen 

 several regions have already been distinguished ; and 

 although they constitute one continuous Avhole, it will 

 be convenient to sj)eak of the sternum (fig. 36, st. XIX), 

 the tergum {t. XIX), and, the 2:>leura {iil. 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 

 ejnmeron (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 (gii. 12), a section of the flexor {fm) and extensor 

 {em) muscles, and of the alimentary {hg) and vascular 

 {s.a.a, i.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 E, cx.p), followed by a long cylindrical second 

 joint, the basipodite {h.p), and receives the name of pro- 

 topodite. At its free end, it bears two flattened narrow 



