THE DECAPOD CRUSTACEA. 163 



and have also mentioned that other possible division which 

 places the prawn as a typically swimming animal, in oppo- 

 sition to the creeping crab and lobster. Accepting this last 

 division, we find that the swimming Decapods, or Natantia, 

 have the following characteristics in common : The body is 

 always more or less compressed, as is also the rostrum. The 

 abdomen is well developed, its first segment is not markedly 

 smaller than the rest, but the second is usually very well 

 developed. The antennae have a five-jointed peduncle and 

 a large scale. The thoracic limbs are slender, are all seven- 

 jointed, and only in rare cases is the first better developed 

 than the others. Usually more than one pair are furnished 

 with chelae, and the penultimate segment is attached to the 

 antepenultimate by one fixed point or fulcrum only, so that 

 it swings less easily than in the Eeptant Decapods where 

 there are two fixed points. The abdominal appendages are 

 used for swimming. When the female carries the eggs 

 about with her, which does not invariably happen, the 

 second pair of swimmerets have a brood-lamella attached 

 to them ; this is seen in the common prawn (Palcemonji 

 Examples of Natant Decapods are shrimps and prawns, of 

 which there are many kinds. Our British forms are all 

 relatively small, but some tropical prawns attain a length of 

 nearly a foot. Most are more or less social, and are found 

 swimming in shoals. 



With these swimming Crustacea are contrasted the 

 Reptantia, which have the following characters : The body 

 is depressed, with a flattened rostrum, or without a rostrum. 

 The abdomen is sometimes well developed and sometimes 

 reduced, but its first segment is always distinctly smaller 

 than the others. The peduncle of the antennae is reduced, 

 and the scale is sometimes absent. The thoracic limbs are 

 strongly developed, are usually six-jointed, and the first is 

 the largest. The penultimate joint is attached to the ante- 

 penultimate by two fulcra, or fixed points. The swimmerets 

 are always more or less reduced, and in the female always 

 carry the eggs. 



It might be supposed that the Reptant Crustacea could be 

 sharply divided into two sets the crabs and lobsters but 

 we shall find that there are many transitional forms. Our 

 British forms are typically larger than shrimps and prawns, 



