THE LOBSTER AS A TYPE OF CRUSTACEA 9 



Tergum 



attached, wide apart, on the under-side. The arched 

 upper part of the ring is known as the tergum, and 

 the more flattened under-part as the sternum. On 

 each side the tergum overlaps the sternum, and 

 hangs down as a side-flap, or pleuron. On the 

 upper side of the abdomen 

 the terga of the somites 

 overlap, the front part of 

 each being pushed under 

 the tergum in front when 

 the abdomen is straight- 

 ened, and only exposed to 

 view when the abdomen is 

 bent. Below, the sternum 

 of each somite is seen to 

 be only a narrow bar, con- 

 nected with those in front 

 and behind by soft mem- 

 brane, and there is no over- 

 lapping. At the sides the somites are connected 

 together by hinge-joints, which allow them to move 

 only in a vertical plane. Thus the abdomen can be 

 straightened out or bent downwards and forwards, 

 but cannot be moved from side to side. In life the 

 Lobster can swim backwards through the water by 

 vigorously flapping the abdomen. 



The carapace which covers the upper side of the 

 head and thorax is not formed, as might be thought, 

 simply by the terga of the somites becoming soldered 



Pleuron 



Appendage 



FIG. 2 ONE OF THE AB- 

 DOMINAL SOMITES OF THE 

 LOBSTER, WITH ITS APPEN- 

 DAGES, SEPARATED AND 

 VIEWED FROM IN FRONT. 



(From British Museum 

 Guide.) 



