22 CRUSTACEA. 



are Intromittent or clasping organs in the male (see 

 Fig. 7). and therefore appropriately developed. The 

 second and third pairs of appendages in the male also 

 have an additional small lobe on the inner side of the 

 larger inner lobe (Fig. 7). 



Hearing in mind the general plan of structure discov- 

 erable in the lobster's abdomen, the scholars may pass 

 to the cephalothorax. The five pairs of appendages 

 in front of the SwimmeretS arc usually called the walk- 

 ing-legs (Fig. 8, />'. e i-e 5). This number has given 

 the name of Decapoda, or ten-footed Crustacea, to the 

 order. The last four pairs of walking-legs — counting 

 always from the anterior end backwards — have seven 

 sections, and the first pair six sections : the fifth and 

 fourth pairs terminate in short spikes (.v). Those 

 of the fifth pair are well suited for being used to 

 push and shove the body ahead. The pliability of the 

 terminal joints secure the points of the sections from 

 slipping, and allow the legs to act against them at any 

 angle in shoving the body forward without renewing 

 the hold, until the whole length of the leg and the full 

 force of one step has been economized. The terminal 

 sections or spikes of the fourth pair arc larger, more 

 sharply pointed, and thrown out forwards and sidewise, 

 They are hooked into the surface of the ground, and 

 used to drag the body and lift it at the same time. The 

 white worn points of the spikes in both of these legs, 

 and the positions which they can be made to take by 

 manipulation, show how they are used. 



At the base of each of the spikes, on the last pair of 

 legs, there is a little spine (v). which is really a down- 

 ward prolongation of the sixth section. It can be felt 



