LOBSTERS , CRAYFISH,\ ETC. 
269 
capable of being folded back upon the penultimate. The common shrimp ( Crangon 
vulgaris ) occurs in shallow water on sandy coasts of temperate countries of the 
Northern Hemisphere. Its colour is a speckled grey, corresponding closely with 
that of the sandy sea-bottom upon which it lives, and in which it buries itself when 
threatened with danger. To escape the vigilance of fish, shrimps resolutely keep 
themselves hidden during the day, but come forth at night to hunt for food. The 
presence of this they perceive by means of iscent, since a blind shrimp will find food 
as quickly as an uninjured one. A second British species is Allman’s shrimp 
{Crangon allmani), abundant in deep water in the Irish Sea and on the west 
coast of Scotland. It may be at once distinguished by the presence of two fine 
keels on the upper side of the sixth segment of the abdomen. Both have a short 
rostrum and no spines on the carapace; but some of the other members of the 
family have crests of spines on the carapace, and sometimes a largish rostrum as in 
the Arctic Sclerocrangon boreas. In Rhynchocinetes typus, from the South Pacific, 
this rostrum is not only large but movably jointed to the carapace. The section 
Monocarpinea differs from the last in having the first and second trunk-limbs 
completely chelate, and the second pair larger than the first. To this section 
belong a number of fresh- and salt-water forms, and amongst them the Palcemonidce 
or prawns. The general form of the body is shown in the figure of the common 
prawn ( Leander serratus). I11 its native haunts the prawn is nearly invisible, 
being almost colourless, translucent, and marked merely with streaks of various 
tints. In the rivers of tropical countries occur prawns ( Palcemon ) rivalling lobsters 
in size, and remarkable for the length of their pincers, which may exceed that of 
the body. Among the largest are P. jamaicensis from the West Indies and Central 
America, and the Indian P. lar, so much esteemed when cooked as a curry. Also 
belonging to the same section is the family Atyidce, containing a few genera such 
as Atya and Caridina, found in both Eastern and Western Hemispheres in fresh¬ 
water streams and lakes. In Atya the trunk-limbs are curiously constructed, the 
first two pairs being short and subequal with the two fingers of the pincers tipped 
with a long tuft of hairs. The remaining three pairs, of which the first is much the 
