Vlll INTEODUCTION. 



of eight segments the last of which is movable on the rest ; 

 and each of these eight segments bears a pair of appendages. 

 The appendages of the last five of these are the ambulatory and 

 prehensile legs. Each of these consists of an exopodite and endo- 

 podite ; the former is quite rudimentary, the limb being consti- 

 tuted by the endopodite. The endopodite of each limb consists of 

 seven joints the basal joint is called the coxa or coxopodite ; the 

 second is the basus, basos, or basipodite ; the third is the ischium 

 or ischiopodite ; the fourth is the meros, merus, or meropodite ; 

 the fifth is the carpus or carpopodite ; the sixth the propodos or 

 propodite ; the seventh and last is the dactylos, dactylus, or 

 dactylopodite. Attached to the coxopodite is a branchial appen- 

 dage the epipodite. The first three of these five posterior 

 thoracic appendages take the form of pincers or chelce ; the last 

 two end in simple pointed claws ; the chelate form is brought 

 about by the production of the propodite beyond the point of 

 insertion of the dactylopodite into a slender process extending 

 as far as the extremity of the latter. In these chelate thoracic 

 appendages the meropodite is frequently designated the arm, the 

 carpopodite the wrist, the propodite the hand, the distal prolon- 

 gation of the latter the immobile finger, and the dactylopodite 

 the mobile finger. The three anterior pairs of thoracic appen- 

 dages are the onaxillipedes or foot-jaws. The last of these has an 

 inner division or endopodite very similar in appearance to the 

 last two pairs of ambulatory legs, and having the same number 

 of joints : its exopodite, however, is much larger than in the 

 case of the ambulatory limbs, consisting of a long compressed, 

 many-jointed palp ; the epipodite is similar to those of the 

 ambulatory legs. In the second pair of maxillipedes the endo- 

 podite is much shorter and the joints more compressed than in 

 the third. The first pair of maxillipedes is constructed on a 

 different plan ; the endopodite takes the form of an ovate hair- 

 bordered plate ; the exopodite is slender and palpiform, and has 

 on its outer side a delicate membranous lamella ; the epipodite 

 consists of a membranous lamina without branchial filaments. 



The appendages of the segments of the head or cephalon are 

 of two sorts those attached round the mouth and connected 

 with mastication viz., the two pairs of maxillae and the man- 

 dibles ; and the sensory appendages attached to the front of the 



