INTRODUCTION. xix 



the sternum. In the Macrura, it also forms part of 

 the sternum, but the separation is distinguishable by 

 a free and movable articulation. In the Sessile-eyed 

 Crustacea, the coxa is more laterally situated, and very 

 firmly attached, without being fused to the segment of 

 the body. With few exceptions, it is developed into 

 a broad and scale-like joint, and is so large in the 

 Stegocephalidte that it covers the greater part of the 

 animal. The object of this development is evidently to 

 cover and protect the branchial appendages, when situated 

 beneath the pereion. These scale-like coxae have been 

 considered as parts of the segments of the body of the 

 animal to which the legs belong, and are described under 

 the name of epimera, or side-pieces, by Professor Milne 

 Edwards. 



There is a peculiar tendency in the Amphipoda for the 

 joints of the legs to be produced in a scale-like form. 

 Besides the coxa3, the basis, or second joint of the three 

 posterior pairs of pereiopoda, are almost always so de- 

 veloped. In Orchestia, the males in some species have the 

 carpus and posterior pair of pereiopoda enlarged ; in Podo- 

 cerus and Cerapus, the two anterior pairs have the basis 

 so produced ; but in Sulcator this predisposition appears 

 to reach the culminating point, where it is apparent in 

 almost every joint of the appendages of the head and 

 body. 



The next division of the animal is that which we deno- 

 minate the pleon. It consists of seven segments, as in 

 each of the former divisions, and carries three kinds of 

 appendages. The segments generally resemble those of 

 the pereion, and, like them, carry on each side squamiform 

 coxa3, which Professor Milne Edwards has again mistaken 

 for epimera, or side-pieces, belonging to each respective 

 segment. These are, both in the Amphipoda and Isopoda, 



