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 podobranehke and long exopodites. After the first 

 molt the swimming hairs and seta' which garnish the endopodites are rapidly ev agin ated. 

 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. 6G) the first pair of pereiopods or large 

 cbelipeds are nouprehensile, armed with stout, scattering seta;, of which those seen on 

 the inner margins of the meros 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 large 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 larvse the opposed margins of the large claws 

 are distinctly toothed, and the latter end iu incurved, horny tips. 



There is usually but very little or no difference in the size of the large chelre until 

 after the seventh molt. In the sixth stage the extremities are already provided with 

 numerous tufts of sensory seta? (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 chela3 for crushing and cutting is a gradual process, but 

 is fairly well established in a young lobster 30 to 40mm. in length (plate 8). It rarely 

 happens that both claws are simdar 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 setre close to its articulation Avith the dactyl. 



In later stages (fig. 76, plate 31) the terminal spine becomes reduced and the 

 terminal cluster of serrated set?e 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 



