NATURAI. HISTORY OP AMERICAN LOBSTER. 
227 
limited to the mouth parts and certain adaptations found in the walking legs, further 
details being given in table 4. The history of the big claws is reserved for the following 
chapter. 
MOUTH PARTS. 
In addition to labrum and metastoma, we designate as mouth parts the six pairs 
of limbs which are concentrated about the mouth opening, and which are modified in 
some degree for dealing with the food. 
When the mandibles open, a conspicuous pink fold of fleshy tissue is revealed over- 
hanging the median V-shaped fissure which is the lobster’s mouth (pi. xxxiii). The 
labrum is shield-shaped and compressed in a peculiar manner, being keeled above and 
below on the middle line, with a thin free edge or border, so that it presents two upper 
and two lower concave surfaces. The lower keel by fitting into the slit of the mouth 
forms the upper bound of this opening as it passes into the dorsal wall of the esophagus; 
the fissure is limited below by a soft, round papilla, from the sides of which spring 
a bifurcated “lower lip,’’ or metastoma. The metastoma on either side consists of 
a short strap-shaped blade fitting closely over the convex body of the mandible ; it is 
slightly ridged on the outer side and sparingly sprinkled with setae. Both labrum and 
metastoma are richly supplied with organs which there is reason for regarding as sensory 
buds. The sides of the mouth are formed by rounded swellings of the esophageal wall, 
and are directly continuous with the metastoma below. When the jaws are closed 
and their outer masticatory ridges meet on midline just over the mouth fissure, the 
concave sides of the labrum fit into deep grooves which traverse the opposing man- 
dibular surfaces, and since the groove of each mandible lies below the level of its cutting 
ridge, it is impossible for the lobster to “bite its lip.’’ The V-shaped mouth described 
leads through a very short esophagus directly to the large stomach sac. All of the 
mouth parts which succeed the mandibles are thin and leaf-like up to the somite vii; 
and all conform to their outer convex surfaces. 
The six pairs of appendages which are concentrated about the mouth are abundantly 
supplied with sense organs, and are charged with a variety of functions, the most obvious 
of which are handing the food along to the mouth and mincing it in the course of passage ; 
that they further serve as organs of the chemical sense and of touch more or less com- 
pletely is not to be doubted. 
The mandibles of the adult lobster (fig. 7, pi. xxxv) are in form like hinged double 
doors set in front of the mouth, and so hung to the cephalothorax of the animal that they 
are capable of swinging only a little way in or out, or toward and away from the middle 
line. The body of the mandible, which probably represents the coxa of a typical limb, is a 
triangular convex bar, with a very oblique axis of articulation corresponding to its long 
anterior side; the opening tendon of the abductor mandibuli muscle is inserted on the 
anterior border, near the outer socket and exerts a pull sufficient to open the “door.” 
The posterior border bears at a more favorable point near its middle, a long tendon, 
from which fan out the fibers of the powerful adductor mandibuli muscle (see p. 242). 
These muscles arise from the inner surface of the carapace on either side in front of the 
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