260 BULLETIN OF THE 



The abdomen is relatively no larger than in adult specimens, and is bent 

 underneath against the sternum. The telson consists of a single piece, with a 

 faint indication of the median suture of the adult. After the next moult the 

 diagonal sutures appear (Fig. 8) and the telson assumes the polymeric charac- 

 ter of the adult.* The posterior border of the telson is fringed with long and 

 delicate setse. 



The first pair of antennae (Figs. 3, 6 I) have an enormously developed basal 

 (auditory) segment (Fig. 3 a), followed by a three-jointed peduncle bearing two 

 flagella (b, c), of which the outer (c) is the longer. 



The. second pair of antennae are very long (longer than in the adult), com- 

 posed of about thirty-four segments. 



The oral appendages have almost exactly the structure of the corresponding 

 appendages of the adult, which are represented on Plate III. Figs. 13-18. 

 The mandibular palpus has attained its perfect, tri-articulate structure, and is 

 as large in relation to the size of the body of the mandible as it is in the full- 

 grown specimens. This is remarkable when one considers its rudimentary 

 form in the last zoea-stage (Plate II. Fig. 8 a). 



The chelipeds are longer than in the adult (Fig. 11). One (commonly the 

 right) is larger than its fellow. Like the four following pairs of thoracic legs, 

 they are beset with setse on either border. 



The ambulatory pairs are also longer than in the adrdt. The dactylus 

 (Fig. 4) is armed with hooks like those in the full-grown specimens (Fig. 19), 

 but it is much more slender. 



The posterior thoracic legs are rudimentary and terminated with a didactyle 

 claw (Fig. 5), as perfectly formed as in the adult. 



The second, third, fourth, and fifth abdominal segments bear peculiar two- 

 branched appendages (Fig.' 10), furnished with long, feather-like setae. The 

 large number of individuals which I examined (which it is fair to presume 

 included both sexes) showed no difference in the number and shape of the 

 appendages of the abdomen. In the adult these appendages take on a very 

 different form in both sexes. In the female they become one-branched, slender 

 organs (Fig. 20), and the pair belonging to the second segment disappears.t In 

 the male only one pair persists (on the second segment, Fig. 21) ; here the 

 second branch exists as a rudiment (a). The series of changes through which 

 the abdominal appendages of the young are converted into the adult form I 

 was unhappily unable to follow. In specimens from South Carolina, 6 mm. 

 across the carapace, the external sexual characters are already acquired. 



My specimens underwent but little change, before I left Newport in Septem- 



* Milne Edwards thought that the sutures in the telson of Porcellana showed that 

 it was formed by a consolidation of the seventh abdominal segment with a pair of 

 appendages belonging to the same segment (Histoire Naturelle des Crustaces, II. 

 p. 249. 1837). It is plain from the development that this theory of the structure 

 of the telson is false. 



+ In the adult females of many species of Porcellana the first, second, and third 

 abdominal segments are destitute of appendages. 



