TEN-LEGGED CRUSTACEA 415 



The colour is produced by various pigments (green, brown, blue, and 

 red), and the colour of the animal depends upon which of these tints 

 predominates. In the process of cooking all the pigments are changed 

 to red, the characteristic colour of all boiled crustaceans ("red as a 

 boiled lobster"). 



iv. Organs of Sense. 



1. Even while resting in its hiding-place (see Section 2) the crayfish 

 usually allows its antennae to protrude. They are moved to and fro, 

 apparently in play, but really for the purpose of reconnoitring the 

 ground, to see whether there may be a prey to be captured by a 

 rapid surprise attack or an enemy lurking in ambush (see Section C, ii.). 

 Of these sensitive tactile organs, the crayfish, like all crustaceans, 

 possesses two pairs. Those of the first, or inner, pair each carry two 

 short flagellate appendages, the " antennules," and contain the organs 

 of smell; while those of the second, or outer, pair are each provided 

 with one similar, but much longer, appendage, the " antennae." Upon 

 the first of the three basal joints of the antennules (protopodite) is 

 found a small vesicle, to which naturalists also assign a sensory function ; 

 but it is not- decided whether it serves as an organ of hearing, or merely 

 for the purpose of maintaining the animal's balance. 



2. At the place where the carapace (see Section B, i.) is prolonged 

 forwards into what is known as the beak or " rostrum " are placed the 

 large eyes, which are constructed on exactly the same plan as the 

 compound eyes of insects (see p. 316). Being placed on stalks movable 

 in all directions, they enable the animal to survey a wide circuit (food, 

 enemies), and even to look backwards, which is of great advantage to the 

 creature during its retrograde movements (see Section vi., 2). During 

 rest the eyes are laid in pits by the side of the rostrum, which at the 

 same time protects them from injuries in front. 



v. Organs of Mastication and Digestion. 



The crayfish is an omnivorous creature, feeding by preference on 

 worms, snails, fish, frogs in short, all kinds of animal substances. In 

 case of necessity, or for the sake of variety, it also consumes the roots 

 and young shoots of water-plants, and when very hard pressed even 

 carrion. Being of very voracious habits, the crayfish avoids rapidly- 

 running waters, since these are usually poor in food. 



1. The crayfish is not able, like a fish, to swallow its prey whole, for 

 its mouth, which is placed on the ventral surface of the body, is incapable 

 of dilatation, and represents merely an opening in the rigid armour. 



