NATURAI. HISTORY OF AMERICAN LOBSTER. 229 



The third and last pair of maxillipeds are similarly curved and conform perfectly 

 to the typical limb, with the exception of a fused third joint between ischium and basis. 

 (Fig. 5, pi. XXXVI, X.) The three terminal segments of this appendage are flattened and, 

 as commonly carried, crooked downward upon the longer and more modified meros 

 and ischium. The latter podomeres are curved upward and outward, are three-sided, 

 and, like the former, bear double fringes of dense setae which are used, among other 

 purposes, as cleaning brushes (see p. 179). In place of the upper or inner fringe, how- 

 ever, the trihedral ischium is provided with a serrate crest, or row of about twenty 

 closely set "incisor" teeth. These tooth-like spines increase in size distally and end 

 over the joint in a strong curved fang. They work on the principle of an old-fashioned 

 nutcracker, but in this case with toothed jaws which are very effective in cutting the 

 coarser pieces of food delivered by the slender claw feet before they are passed on to 

 the smaller mouth parts. The first three segments of this limb are closely appressed 

 and quite flat where they meet on the midline, the coxa bearing two flat and hairy 

 spurs. 



The third pair of maxillipeds are the only really effective "jaw feet," and with 

 the mandibles the only appendages which play an important part in reducing the 

 food. Of the other mouth parts, the maxillae, especially the smaller first pair, and 

 the second maxillipeds without doubt help in the mincing process to which the food 

 is subjected, but their chief function, as in the first maxillipeds, is without doubt sen- 

 sory and for passing the food up to the mandibles. When the latter have finished 

 their work the "grist" is ready for the gastric mill. 



THE SLENDER LEGS. 



The ten thoracic legs, which are designated as the pereiopods in the higher Crus- 

 tacea, consist of the great chelipeds and four pairs of slender walking legs (pi. xxxvin), 

 the first two of which bear weak compound or double claws and the last two end in 

 simple dactyls. 



The successive segments of these limbs move on hinges, a description of which 

 is given in chapter vii, and are actuated by opposing muscles in the typical way with 

 the exception of basis and ischium, in each of which a flexor is absent. (Fig. i, pi. xli.) 

 The basis has but one ventral or posterior extensor, with movement limited to a few 

 degrees of arc, and the ischium two posterior extensors inserted upon two tendons, 

 which are set close together on the margin of the shell at the opening of the meros. 

 Accordingly these limbs can not be flexed at the fourth joint. There are no fused 

 joints in the slender legs, which commonly break between basis and ischium, and are 

 regenerated from this plane. 



Aside from their direct use in locomotion, the smaller pereiopods present a variety 

 of functions, the last pair possessing brushes for cleaning the abdomen (see p. 303), 

 and incidentally serve as picks to steady the animal as it crawls over the bottom. Far 

 more significant, however, are the clusters of sensory setae {s. s., pi. xxxvm) arranged 

 in symmetrical rows on the last two segments of the slender legs. One can count a 



