THE AMERICAN LOBSTER. 
197 
The pereiopods . — The general structure of the pereiopods is shown in the various 
plates illustrating the larval and adolescent history. For a considerable period before 
the time of hatching the great chelipeds can be distinguished by their size. At the 
time of hatching all have prominent podobranchiae and long exopodites. After the first 
molt the swimming hairs and setae which, garnish the endopodites are rapidly evagiuated. 
The first three pairs of pereiopods are subchelate. After the fourth molt (fig. 67, plate 
30) the exopodites are reduced to rudiments and leave no trace in the sixth stage. 
The first pereiopods. — In the first larva (fig. 66) the first pair of pereiopods or large 
chelipeds are non prehensile, armed with stout, scattering setae, of which those seen on 
the inner margins of the ineros and ischium are the representatives of stout spurs 
which are developed in the fourth larva. Both propodus and dactyl end in a strong, 
nearly straight spine, which in the latter joint is conspicuous for its length. 
Autotomy of the lairge chelipeds occurs in the fourth larva, but fusion of the basis 
and ischium is not effected until at least after the fifth stage (plate 33, fig. 96, and 
plate 30, fig. 67). 
In the second, third, and fourth stages the prehensile claw is gradually developed 
(plates 20-23). In the third and fourth larvrn the opposed margins of the large claws 
are distinctly toothed, and the latter end in incurved, horny tips. 
There is usually but very little or no difference in the size of the large chela? until 
after the seventh molt. In the sixth stage the extremities are already provided with 
numerous tufts of sensory setae (compare plates 23-25). In the later adolescent stages, 
when the differentiation of the large claws is complete, these tufts are mostly confined 
to the cutting claws, where they form a dense mat over the toothed margins and 
extremity of the propodus (plates 10-12). 
The differentiation of the chela? for crushing and cutting is a gradual process, but 
is fairly well established in a young lobster 30 to 40 mm. in length (plate 8). It rarely 
happens that both claws are similar in the adult stages (see Chapter ix). 
Second and third pereiopods.-- The structure of these appendages, which agree, 
except in size, is illustrated by fig. 73. In the fourth larva (fig. 74) the chelate struc- 
ture is pronounced and the exopodite is a rudiment. 
Fourth and fifth pereiopods. — The dactyl of these appendages in the first larva 
(fig. 70, plate 31) ends in a very long, nearly straight spine, while the propodus bears 
a characteristic cluster of seta. 1 close to its articulation with the dactyl. 
In later stages (fig. 76, plate 31) the terminal spine becomes reduced and the 
terminal cluster of serrated setae on the propodus is then very conspicuous. In the 
fifth larva the constriction at the proximal end of the ischium is clearly defined and 
the exopodite has disappeared except as a microscopic rudiment (not shown in fig. 76). 
The pleopods. — The second, third, fourth, and fifth pairs of abdominal limbs are 
visible as buds beneath the cuticle of the first larva and emerge after the second molt 
(plates 20, 21). 
The sixth pair of pleopods, which form with the telson the tail-fan, are seen as 
rudiments through the cuticula of the second larva and are released with the third 
molt. 
THE DEVELOPMENT OF THE FIRST PAIR OF PLEOPODS. 
No accurate observations have been made upon the development of the first pair 
of abdominal limbs, which are specially modified in the two sexes. They are the last 
appendages to appear, and their growth and differentiation are very gradual. In the 
