l6 A BIOLOGY OF CRUSTACEA 



which are large and ' feathered * for this purpose, while those 

 members of the Reptantia which swim do so without using their 

 pleopods. 



The Natantia include the prawns and shrimps, but it should be 

 made clear that these popular names have no scientific application; 

 they are applied indiscriminately to various members of the sub- 

 order. In common English usage shrimp is usually applied to 

 members of the genus Crangon, the pink shrimp is Pandalus 

 montagui, and the name prawn is given to members of the genus 

 Palaemon. In North America members of the last genus are called 

 shrimps. 



Three groups can be distinguished within the Natantia. These 

 are the Penaeidea, with the first three pairs of legs bearing pincers, 

 the Stenopodidea, also with three pairs of pincers, but with the third 

 legs stronger than the first two, and the Caridea, which have no 

 pincers on the third legs. The Penaeidea are regarded as the most 

 primitive of these groups, and they are among the earliest fossil 

 decapods, being found in the Triassic deposits of Madagascar. The 

 Caridea are not found until the Jurassic period, while the Steno- 

 podidea are not known as fossils, probably because they are delicate 

 and not easily preserved. 



The really big Crustacea belong to the Reptantia. Lobsters 

 (Homarns species) sometimes reach a weight of nearly forty pounds, 

 and there is a giant Japanese spider crab (Macrocheira kaempjeri) 

 with legs spanning eight feet. 



Dividing the Reptantia into sub groups is a troublesome business, 

 because intermediates between the various proposed groups are 

 common, and much depends on which structures one thinks are 

 important. For our purpose we can divide the group into the usual 

 four sub groups : the Palinura, Astacura, Anomura and Brachyura. 

 The first two are sometimes grouped together as the Macrura, the 

 big tails, but this is also a term which is sometimes applied to these 

 together with the Natantia, so that its use is generally to be avoided, 

 although sometimes useful as a descriptive term. 



The Palinura differ from the true lobsters and crayfish (Astacura) 

 in not having a pointed rostrum and in lacking the characteristically 

 enlarged pincers. The spiny lobsters, Palinurus and Panulirus, 

 reach a considerable size; there is an authentic record of an 

 American species reaching a weight of 26 pounds, and an uncon- 

 firmed record of one weighing 34 pounds. 



True crabs, or Brachyura, are the most numerous members of 



