342 SYNOPTIC COLLECTION. 



side like a saw and provided with short hairs. The eyes 

 are prominent, flattened, and black in color. The anten- 

 nae are the most conspicuous organs, the scales of the 

 second pair extending beyond the rostrum. These organs 

 and the mouth parts are fringed with hairs, giving an ele- 

 gant appearance to Penaeus. The first two pairs of tho- 

 racic legs are much smaller than the remaining three 

 pairs and are provided with claws. 



The abdomen tapers to a sharply pointed telson and 

 carries good-sized swimmerets that serve as a protection 

 for the masses of eggs. 



Most of the groups of Crustacea, although composed 

 largely of marine forms, yet have also fresh-water and 

 terrestrial members, and it is interesting to note that one 

 species of Penaeus (P. brasiliensis Latr.) is often found 

 in brackish waters, and even ascends streams to points 

 where the water is nearly or quite fresh. 1 



The fresh-water prawn, Palaemon carcinus Oliv. (No. 

 840, on upper shelf) is remarkable for the extreme length 

 of its antennae. It is indeed surprising that organs of 

 such delicacy and length can be of use. 



The second pair of walking-feet in this genus are the 

 arms and claws, and these are also greatly prolonged. 

 The first pair, according to Gosse, 2 are used as brushes 

 to clean the ventral side of the thorax and abdomen. 

 The carapace is short and without a median suture ; it is 

 armed in front with an elegant curved rostrum having a 

 double notched edge. In the species Palaemon serratus, 

 Leach found that the point of the rostrum was notched 

 in three thousand specimens, while the notch was want- 

 ing in only two specimens, 3 certainly a good example of 

 the persistency of a structural character. The eyes of 

 Palaemon are small and on slender stalks. The large 

 swimmerets of the abdomen show finely in No. 840. 



1 Stimpson, Ann. Lye. Nat. Hist. N. Y., X, 1874, p. 133. 



2 Quoted by Adam White, Pop. Hist. Brit. Crust., 1857, p. 131. 



3 Malac. Podoph. Brit., no. 15, 1817. 



