THE AMERICAN LOBSTER. 1!)5 



is more rudimentary. The arttennular nerve was traced by Sars toward the end of 

 the appendage, where it appeared to divide into two branches. 



Second antenna. — The outer antenna of the first larva (fig. 45, plate 27) consists 

 of three parts, which are represented in the adult (fig. 118) — a two-jointed stalk, a 

 segmented endopodite, and a scale like exopodite, terminating in a sharp tooth and 

 bordered on its inner margin with long plumose hairs. 



This is the first of the naupliar appendages to become bilobed (cut 30). In an 

 embryo four months old, fig. 108 (compare fig. 77), the cuticular molt shows the traces 

 of only a few terminal setae on either branch. 



The progressive changes in the second antenna during the first five larval periods 

 (figs. 45-49) consist mainly in the reduction of the scale and its feathered setae, in the 

 rapid growth and segmentation of the slender endopodite, and differentiation of the 

 latter into stalk and flagellum proper, and in changes in the stalk or protopodite. In 

 the fourth larva (fig. 48) the first segment (coxa) : of the latter bears a prominent tubercle 

 (already conspicuous in the third stage), on which the duct of the excretory organ 

 opens, while the second segment or basis is divided by oblique constrictions into three 

 parts, as in the adult lobster. The external division bears, next the articulation of 

 the scale, a stout spine which grows with the gradual reduction of the exopodite until 

 it finally nearly equals the latter in length. The terminal seta; of the flagellum are 

 rapidly reduced and are barely recognizable in the third larva. 



The mandibles. — The jaws of the first larva (fig. 54) consist of a stout basal portion, 

 with toothed, indurated, coronal surfaces, meeting on the middle line opposite the 

 mouth, and of a slender, three-jointed palpus, which terminated in the specimen fig- 

 ured in a single strong seta. The cutting edges are asymmetrical, and at the anterior 

 angle there is a stout, variously toothed process which is separated from the rest of 

 the coronal surface by a deep groove. As Professor Smith observed, this is most 

 prominent on the left side. 



In the third larva the asymmetry of the coronal surfaces is even more striking, 

 particularly in the toothed process on the outer side at the anterior end. On the left 

 side this forms a widely overlapping fold, which carries three sharp teeth. The process 

 of the right side is smaller, but had also three teeth in the specimen examined. 



In the fourth larva the mandible is deeply cleft by a wide groove, as in the adult 

 (figs. 55-57), and the brush-like palpus folds over the cutting margin into the fossa. 



In the fifth stage the mandibles are still asymmetrical, and the teeth are no longer 

 sharp but tubercular, the smaller being at the posterior angle. The stout-toothed 

 process of the first larva remains as a blunt tubercle on the left side, where it was 

 most prominent, but has disappeared from the right side. 



First maxilla. — The metamorphosis of this appendage from the larval to the adult 

 condition is relatively very slight. It consists in the first larva (fig. 51) of coxa, basis, 

 and a one-jointed endopodite. All are armed at their extremities with setae of various 

 kinds — slender, sensory hairs on the outer segment, a double row of stout masticatory 

 teeth on the basis, and a cluster of less regular, distinctly serrated, stiff bristles at 

 the extremity of the coxa. 



1 The following abbreviations for the segments of the decapod limb will be used: (1) coxa = cox- 

 opodite; (2) basis — basipodite; (3) iscft.iitm=iscliiopodi.te; (4) ?neros = meropodite; (5) carpus = carpo- 

 dite; (6) propodus = propodite ; (7) dVictyZ = dactylopodite. 



