THE DECAPOD CKUSTACEA. 159 



and the last three pairs are simple, ending in sharp claws. 

 The legs will be found to differ a little in the different 

 kinds of prawns, but are always very different from those of 

 lobster or crayfish. 



The tail is remarkable for the great development of the 

 five anterior pairs of swimmerets, as compared with those of 

 Nephrops. Most of the above points should be readily 

 made out from the accompanying figure. 



If from the prawn we turn to the crab, we find well- 

 marked differences from both prawn and lobster. It is only 

 possible to point out some of these differences. The carapace 

 has been, as it were, strongly flattened out, and in the 

 process the rostrum has disappeared, and the relative posi- 

 tion of eyes, antennae, and antennules altered enormously. 

 Prawn and lobster swim rapidly, and as they swim their 

 long feelers, their freely movable eyes, make them fully 

 aware of their surroundings, while their vigorous tail strokes 

 remove them instantly from the dangers of which those 

 keen sense-organs give them notice. But the crab only 

 moves slowly ; it only requires to be made aware of its 

 immediate surroundings ; it is often content to offer a 

 passive resistance to foes. Therefore its antennae are 

 shorter, less prominent, and capable of more or less com- 

 plete retraction ; the eyes are sunk in orbits which protect 

 them from harm even if they also limit the field of vision. 

 The gills are more efficiently protected, and the parts about 

 the mouth are much modified. Again, while in prawn and 

 lobster more than one pair of legs bears terminal forceps, in 

 the crab it is only the first pair which is thus modified ; the 

 others are simply pointed, and used for locomotion only. 



Let us look now at these points in a little more detail. 

 The carapace, or shield, of the crab is in essence similar to 

 that of prawn and lobster, and shows a similar division into 

 regions, but, besides being flattened and expanded laterally, 

 it is inturned at the anterior and lateral margins. This is 

 readily seen, and the change may be expressed in a rough 

 metaphor by saying that a crab's shield is like that of a 

 lobster which has been crushed flat. As a result in part of 

 this crushing, we find that the lateral area which in the 

 lobster or crayfish forms the vertical gill-cover has here 

 become horizontal, and is separated from the remainder of 



