298 THE INVERTEBRATA 



young stages of various other crustaceans, is known as the dorsal organ 

 or neck gland. It is used by cladocera and conchostraca for temporary 

 fixation. In other cases its function is not known. Possibly the organs 

 to which this name is given are not all homologous. They must not 

 be confused with the "neck organ" of branchiopods (see p. 307). 



Of the appendages or limhs of the Crustacea, the first, or antennule, 

 is a structure sui generis, not comparable in detail with any of the others. 

 Typically it is uniramous, and though in many of the Malacos- 

 traca it has two rami, these are probably not homologous with the 

 rami, described below, of other appendages. The remaining limbs 

 may all be reduced to one or other of two types — the " biramous " limb 

 usually so-called, to which most of them more or less clearly conform* 

 2ind the phyllopodiiim, to vfhxch. belong the trunk limbs of the Branchio- 

 poda and some other appendages, chiefly maxillules and maxillae 

 and notably the maxilla of the Decapoda. The name by which the 

 first of these types is generally known refers to the fact that limbs 

 which best represent it fork distally into two rami. Since, however, 

 the phyllopodium possesses the same two rami, and bears them, 

 though not as a distal fork, yet in the same way as a great number 

 of limbs of the first type, it is well not to use a name which might 

 imply that there is a constant difference in respect of the rami between 

 the limbs of the two types. We shall therefore call the first type the 

 stenopodium^ referring to its usually slender form (Gk. cnevo^^ 

 narrow). 



In the stenopodium (Figs. 211 D-G, 212), the two rami — an inner 

 endopodite and an outer exopodite — are set upon a common stem, the 

 protopodite. In many cases the protopodite bears also, on its outer 

 side, one or more processes known as epipodites (Fig. 212, ep.). In 

 limbs in which the type is most perfectly developed the two rami are 

 subequal and are borne distally upon the protopodite (Fig. 211 G), 

 but in most cases the endopodite is the larger, and forms with the 

 protopodite an axis, the corm, on which the exopodite stands laterally 

 (Figs. 211 E, F, 212, 272). In a few instances the exopodite is the 

 larger. 



The protopodite most often has two joints, a proximal coxopodite 

 and a distal basipodite. In certain cases, however (as in the antenna 

 of the Mysidacea and Asellus, the last three thoracic limbs of the 

 Stomatopoda, certain swimming limbs of the Branchiura (Fig. 

 243 B), and less clearly in many other instances), a basal joint, the 

 precoxa or pleiiropodite , precedes the coxopodite ; moreover the basi- 

 podite may be divided into two joints, the probasipodite, which then 

 usually bears the exopodite, and the metabasipodite or preischiopodite . 

 This condition is seen most clearly on the thorax of the mala- 

 costracan genera Anaspides and Nebalia (Fig. 211 E), where the 

 two components of the basipodite are separate in some limbs and 



