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 aijpendages 

 (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 (t. XIX), and, the pleura {pi. 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 Qig) and vascular 

 {s.a.a, i.a.a) sj^stems. 



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

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



