158 INTRODUCTION TO ZOOLOGY chap. 



tliorax are difficult to distinguish, for, dorsally at any 

 rate, the separate segraents of the exo-skeleton are fused 

 together into one shield-like piece covering the back and 

 hanging down laterally. This dorsal shield is called the 

 carapace. Amongst these forms are a number of genera 

 characterised by having the last five pairs of legs of the 

 thorax enlarged, to form the "ambulatory" appendages or 

 " walking legs," for they alone are used in locomotion over 

 the ground. Because of this special characteristic they are 

 known as the Decapod (ten-legged) Crustacea ; here belong 

 all shrimps, crayfish, lobsters, and crabs, the first three 

 being examples of the Long-tailed Decapods, and the last of 

 the Short-tailed Decapods. 



All Decapods have compound eyes borne on short mov- 

 able stalks, of the same type as that illustrated and explained 

 on pages 208 and 209. 



Order I. : Decapoda 



Long-tailed Decapod Crustacea 



Type : The Common Prawn (Palaemon serrattts). 



The Common Edible Prawns live in shoals in rather deep 

 water off our rocky sea-coasts, sometimes approaching the 

 shore in large numbers. They are more common on the south 

 and west coasts than on the east. 



When alive their colour is a pale greenish-grey, very 

 transparent and striped transversely with red or brown. 

 When boiled they turn a bright red, thus differing from 

 shrimps, which become merely a pinkish-brown under the 

 same treatment. A full-grown prawn is about 4 inches long 

 from head to tail, and the body is laterally compressed. Its 

 general shape is shown in Fig. 95. 



The exo - skeleton consists of the carapace 

 skeMon '"^^^''^ covers the head and thorax dorsally and 

 laterally ; behind this are six separate shelly seg- 

 ments covering the back and sides of the first six abdominal 

 segments, and finally a pointed tail-piece or telson. The carapace 

 is extended forward as a strong-toothed beak or rostrum. 

 Ventrally, a thickened band or bar of the " shell " runs across 

 each segment ; these bars are distinct in the abdomen, but 



