414 TEXT-BOOK OF ZOOLOGY 



which extends across the dorsal carapace. (The crustaceans in which 

 the cephalo-thorax is wholly or partially covered by such a carapace 

 form the large division of the so-called Armoured Crustaceans, or 

 Thoracostraca. The carapace goes by the familiar, or kitchen, name 

 of " head.") From the cephalo-thorax the somewhat narrower abdomen 

 (" tail ") is distinctly separated. Both divisions pf the body carry a large 

 number of limbs, varying considerably in structure, according to the 

 different functions assigned to them. 



ii. Exoskeleton, or " Shell." 



All the parts of the body are encased in a solid armour, which serves 

 as an important protection to the animal against enemies and injuries,. 

 Like that of all arthropods, it consists of chitin (see p. 308), but acquires 

 great firmness by the deposition in it of calcareous salts. (Hence, the 

 name "crustacean." Eemove the salts by soaking the shell in dilute 

 hydrochloric acid ; of what does the soft flexible portion which remains 

 consist ?) ,The chalk is derived (as in molluscs, see vineyard snail) from 

 the food, and separated by the blood simultaneously with the chitin 

 over the whole surface of the body. Hence a third, condition to be 

 satisfied by the water to render it suitable for the habitat of the crayfish 

 is that it contain calcareous salts. Like all arthropods, the growing 

 crayfish is obliged to cast its shell periodically in order to form a more 

 capacious one (see p. 310). The new envelope, however, is at first very 

 soft, and the animal, being, therefore, quite defenceless during this 

 condition, carefully endeavours to hide itself until its armour has 

 regained its hardness, which takes place pretty rapidly (in from eight 

 to ten days) ; for even before the moult a store of calcareous material 

 has been accumulated in the body in the form of two lenticular masses 

 known as " crabs' eyes," to which formerly peculiar healing properties 

 were attributed. They are found embedded in the lateral walls of the 

 stomach, and during the moult are dissolved and carried into the blood. 



Hi. Colour. 



The colour of the animal also has its seat in the shell of the animal, 

 and adapts itself in a high degree to the bottom of the water it inhabits. 

 Thus, crayfish which live on the light-coloured bottom of clear lakes are 

 of a light greenish colour (thus, in a lake in Poland, the waters of which 

 are of crystalline clearness, "light green, nearly white" crayfish are 

 found) ; on the other hand, those living in the dull waters and on the 

 dark bottoms of rivers vary from brown to almost black. (Explain these 

 phenomena by comparison with the ringed snake and other animals.) 



