XII 



CRUSTACEA 



161 



known as the swimmerets, whilst the sixth, the last pair, is 

 broadened and flattened, and turns back to lie on either side of 

 the tail segment, thus forming the tail "fin." 



On the sea-bottom the prawn moves along on 

 s> its " walking legs," but when swimming these 

 appendages are used little, if at all. A gentle forward- 

 swimming motion can be attained by the movements of 

 the swimmerets, but when startled, the prawn will dart 

 suddenly backwards through the water. This it does by 

 violently and suddenly lashing its whole tail forward under 

 its body; this drives the water in front of it, and the reaction 

 causes the body to jerk backwards. 



The gills or respiratory organs are hidden away 

 L in a cavity lying below the side " flaps " of the 

 carapace, which should be cut away on one side, so as to 

 expose the gills 

 (Fig. 97). There 

 will then be seen 

 seven little white 

 elongated plates, 

 decreasing rapidly 

 in size from the 

 largest attached 

 to the body -wall 

 just above the last 

 walking leg to 

 the very minute 

 little anterior one, 

 attached at the 

 base of the second 

 " jaw foot." These 

 white plates are the gills in which the blood circulates, and 

 in order to keep the blood aerated it is necessary that a 

 constant current of water should pass over them. That this 

 occurs can be demonstrated by putting a little" carmine, or 

 other easily visible but harmless powder, into sea-water with 

 a living prawn, when the current will carry it in at the back 

 margin of the side of the carapace, and out at the front 

 margin. This current is maintained to some degree by the 

 backward movement of the creature through the water, or, when 

 it is at rest, by the movement of a little flat horny plate o* 

 VOL. I M 



FIG. 97. View of a Prawn, with one side of the 



Carapace cut away. 



c, Cut edge of carapace ; s, scaphognathite or plate of the 

 2nd maxilla. 



