ANNULOSA: CRUSTACEA. 28 1 



ing only in the fact that the exopodite and endopodite are 

 much narrower, and the former is undivided (fig. 136). The 

 last thoracic somite immediately in front of the abdomen 

 carries a pair of the walking or ambulatory legs, each consisting 

 of a short basal piece, or " protopodite," and of a long jointed 

 "endopodite," the "exopodite" not being developed. The 

 next thoracic segment carries another pair of ambulatory limbs, 

 quite similar to the last, except for the fact that the protopo- 

 dite bears a process which serves to keep the gills apart, and 

 is termed the " epipodite." The succeeding segment supports 

 a pair of limbs similar to the last in all respects, except that 

 its extremities, instead of being simply pointed,- are converted 

 into nipping claws or "chelae." The next segment of the 

 thorax carries a pair of chelate limbs, just like the preceding ; 

 and the next is furnished with appendages, which are essen- 

 tially the same in structure, but are much larger, constituting 

 the great claws. The next two segments of the thorax, and 

 the segment in front of these (by some looked upon as belong- 

 ing to the head, by others as referable to the thorax), bear 

 each a pair of modified limbs, which are termed " maxillipedes," 

 or "foot-jaws." These are simply limbs with the ordinary 

 structure of protopodite, exopodite, endopodite, and epipodite, 

 but modified to serve as instruments of mastication, the hind- 

 most pair being less altered than the two anterior pairs (fig. 

 T 37> 3)- The next two somites carry appendages, which are 

 in the form of jaws, and are termed respectively the first and 

 second pairs of " maxillae." Each consists of the parts afore- 

 mentioned, but the epipodite of the first pair of maxillae is 

 rudimentary, whilst that of the second pair is large, and is 

 shaped like a spoon. It is termed the " scaphognathite," and 

 its function is to cause a current of water to traverse the gill- 

 chamber by constantly bailing water out of it. The next seg- 

 ment carries the biting jaws or " mandibles ; " each of which 

 consists of a large protopodite, and a small endopodite, which 

 is termed the "palp," whilst the exopodite is undeveloped. 

 The aperture of the mouth is situated between the bases of the 

 mandibles, bounded behind by a forked process, called the 

 "labium," or " metastoma," and in front by a single plate, 

 called the "labrum" (upper lip). The next segment bears 

 the long antennae, or feelers (fig. 136, a'\ each consisting of a 

 short protopodite, and a long, jointed, and segmented endo- 

 podite, with a very rudimentary exopodite. In front of the 

 great antennae is the next pair of appendages, termed the 

 " antennules," or smaller antennae (fig. 136, a), each composed 

 of a protopodite, and a segmented endopodite and exopodite, 



