222 
BUI.LETIN OF THE BUREAU OF FISHERIES. 
APPENDAGES. 
The 20 pairs of appendages of the lobster are developed as tubular folds or out- 
growths of the body wall, and consist of ectoderm with mesodermic cores, a rule which 
seems to be broken only in the case of regenerating limbs, where ectoderm appears to 
contribute to the renewal of both muscles and nerves. The order of embryonic develop- 
ment is; (i) Antennules, (2) mandibles, (3) antennae, (4) maxillae and the thoracic 
limbs in regular succession. Four pairs of swimmerets (somites xvi-xix) are released 
together in the second larval stage (fig. 41); the uropods in the third stage (fig. 42) 
and the first pair of pleopods, which are the last to appear, are not usually recognizable 
until the sixth molt or later. 
The eyestalks, which are omitted from the enumeration given above, and the 
antennules are prostomial in origin, while the originally postoral antennae reach a 
position in front of the mouth by the twentieth day, when the compound eyes are dis- 
tinctly lobate. Segmentation in the limbs is a gradual process, constrictions early 
marking future joints, while the division into outer and inner branches begins at the 
apex of the appendage except in the antennules, as noted below. Most parts of the 
adult appendages are recognizable in the first larva, and all, excepting those of the xv 
somite, in the lobsterling. From the fourth stage on through the adolescent period 
the changes are gradual and relatively slight, excepting only those which involve the 
