



-,Q 3 2 CRUSTACEA. 



In figure 52, joints 1, 2, and 3, were united in one, or the first is 

 obsolete. This is a common case. Figure 49 is another example. 



When the branch is wanting, the whole five joints of the main 

 stem are sometimes present, as in figures 44 and 46. But often one 

 or more are not apparent. In figure 48, 1 and 2 are not disjoined. 

 In figure 47, either the 5th, or apical, is wanting, or the joints 3 and 

 4 are united ; the latter, we think the fact, since in some species an 

 articulation crossing this joint is distinctly seen. In figure 45, there 

 are but three joints in all. The numbers on the figures will aid in 

 comprehending these normal relations. 



Buccal area.— The buccal area is a low convex prominence, with 

 the opening of the mouth on the posterior side (see figures on plate 

 75, which will be particularly described on a following page). 



Mandibles or mandibular feet— -The mandibular feet vary in number 

 of joints from one to five. When more than one, the basal is usually 

 called the mandible, and the others a palpus,— a distinction which 

 is without good foundation. The whole is properly a mandibular 

 foot, of which the first joint is laterally prolonged, and constitutes 

 the mandible. 



This mandible has either a simple corneous extremity, Plate 71, 

 fig. 56, more or less pointed; or it terminates in a dentate edge, and is 

 furnished often with a seta on one side near apex (figs. 57-62) * The 

 teeth are in general nearly equal, excepting one or two at one side, 

 which are larger and longer (figs. 60, 61, 61 a), though not projecting 

 beyond the line of the others. The mandible enters the buccal area 

 from one side, and has some lateral play, through the action of power- 

 ful muscles. 



The rest of the organ, when other joints exist, is either simple or 

 two-branched. Figure 57 represents an example with a simple extre- 

 mity, consisting of four joints, or five, including the basal or mandi- 

 bular joint. In figures 58 to 61, there are two branches, the branches 

 proceeding from the second normal joint, as numbered on the plate. 

 In figures 58 and 60, the branches are one-jointed; in figures 59 and 



* On Plate 71, fig. 56, from a Sapphirina; 57 a, b, Calanus, different views; 58, 

 Calanus; 59, Euchaeta; 60, 61, 61a, Pontella; 62, Harpacticus; 63, Oithona. 



