THE CRUSTACEA 



" carapace " so formed really represents the shell-fold, and its lateral 

 extensions, which cover the bases of the legs, may be the pleura of 

 the coalesced somites. Apart from the Copepoda, the only 

 Crustacea in which there is no trace of a shell-fold are the 

 Anostracous Branchiopoda, and perhaps also the anomalous Mala- 

 costracan Anaspides. 



Before proceeding to discuss the true limbs, mention must be 

 made of certain appendages which have sometimes been regarded 

 as homologous with the limbs, but which probably do not belong to 

 that category. In most Malacostraca and in certain other forms, 

 notably among the Copepoda, the posterior margin of the oral 

 aperture is bounded by a fold forming a lower lip (metastoma or 

 hypostoma), usually cleft into two lobes, known as the paragnatlia, 

 which may bear movable terminal lappets. Since there is never 

 any corresponding pair of ganglia on the ventral nerve-chain, or 

 other evidence of the existence of a corresponding somite, there is 

 little ground for the view that the paragnatha are a vestigial pair 

 of limbs. Clans has shown that in Apis the so-called paragnatha 

 are probably the proximal lobes of the maxillulae, and he has 

 suggested a similar connection in the Malacostraca, where, however, 

 an independent origin of the lower lip seems more probable. The 

 upper lip or labrum, already mentioned, is plainly an unpaired 

 outgrowth. 



The terminal segment or telson often bears a pair of processes 

 or rami forming the " caudal furca." These may be multiarticulate 

 filaments as in Apus and a few Cirripedes ; in the Anostracous 

 Branchiopoda, Copepoda, and Leptostraca they are unsegmented 

 rods articulated to the body ; in other cases they may be simple 

 processes of the telson. There seems to be very little reason for 

 supposing that the furcal rami represent limbs, more especially 

 since the telson, as stated above, has not the value of a true somite. 



Limbs : General Morphology. The limbs of Crustacea differ very 

 widely in form and structure, but it is generally possible to trace 

 in them the modifications of a fundamental type consisting of a 

 peduncle, the protopodite (or sympodite), bearing two rami, the 

 exopodite and endopodite. This simple biramous form is seen, for 

 instance, in the swimming feet of the Copepoda (Fig. 3, B), the 

 cirri of the Cirripedia, and the abdominal appendages of the 

 Malacostraca (Fig. 3, A), and in the second and third pairs of limbs 

 in the earliest and most primitive type of larva, the nauplius 

 (Fig. 3, C). As a rule, the protopodite is composed of two segments 

 known as the coxopodite (or coxa) and basipodite l (or basis), but one 

 of these may be reduced or suppressed ; while, on the other hand, 

 Hansen has shown that in some cases a pre-coxal segment can be 



1 By some writers the term basipodite is applied to the protopodite as a whole. 



