VI PENEIDEA AND CARIDEA—-SHRIMPS AND PRAWNS 159 
s — — = ——— 
are described by Alcock * as possessing peculiar secondary sexual 
characters. Thus Parapeneus rectacutus 6 has one lash of the 
first pair of antennae peculiarly bent to form a clasping organ, 
while Aristaeus crassipes has a hook on the end of the third 
maxillipede. In the latter the females have much longer rostra 
than the males, and are in general more powerfully built, so that 
they seem to have usurped the proper functions of the male, and 
probably engage in combats with one another over his person. 
As a general rule the Shrimps and Prawns occur in large 
shoals in the shallow waters of the littoral zone, and they have 
a remarkable power of adapting their colours to the surroundings 
in which they happen to be at any particular moment.” This is 
brought about by the variously coloured chromatophores, which 
contract and expand 
in obedience to a 
stimulus transmitted 
through the eyes of the 
animal. A number of 
the Palaemonidae go 
up rivers into fresh 
water, while one 
family, the Atyidae, 
live in the completely 
fresh water of rivers 
and inland lakes. The 
Peneidea undergo a 
very complete meta- 
morphosis which is 
primitive in respect 
to the order of forma- 
tion of the segments 
: Fic, 106.—Nauplius larva of Penews, sp. x 25. 
from before backwards. (From Balfour, after F. Miiller). 
The larva hatches out 
as a Nauplius (Fig. 106), which by the orderly addition of segments 
1 Loe. cit. p. 150. 
2 Keeble and Gamble, Phil. Trans., Ser. B, exevi., 1904, p. 295. The chromato- 
phores are also directly responsive to light, but the lasting adaptations to colour- 
backgrounds are brought about indirectly, the stimulus being transmitted through 
the eyes and nervous system. The influence of light may also affect the metabolism 
of the animal, the chromatophores being accompanied by a ramifying fatty tissue, 
which disappears if the animal is kept in the dark. 
