TEN -LEGGED CRUSTACEA 



417 



parts.) The two following pairs, or maxilla, are without these hard, 

 masticatory parts, as are also the three last pairs, which have more 

 the character of limbs, and are therefore known as foot-jaws or 

 maxillipedes. Their function consists not so much in dividing up the 

 food as in passing on the morsels conveyed to them by the large pincers, 

 and to prevent them from being washed away by the water. This also 

 explains the large number of these mouth parts, and their increasing 

 size as we pass from before backwards. In the last pair, which covers 

 all the other mouth parts from below, the longest (basal) joint has a 

 very sharp inner edge, which adapts it admirably for scraping off the 

 scales of fish. (As to the importance of the foot-jaws as accessory 

 respiratory organs, see Section vii.) 



5. The complete division or trituration of the food, however, as in 

 grain-eating birds (see Part II., p. 197) and several insects, takes place 

 in the stomach, the walls of which are fortified by blunt-toothed, chitinous 

 masticatory plates rubbed against each other by powerful muscles. 



vi. Organs of Locomotion. 

 1. On its five long pairs of thoracic or ambulatory legs (" decapod 



on stilts (gressorial 



crustaceans "), the crayfish strides along slowly as 

 feet). It never walks backwards unless 

 alarmed. In creeping along, the 

 animal does not as a rule make use 

 of the powerful pincers of the first 

 pair of legs, though these are of help 

 to it as climbing organs in surmount- 

 ing obstacles. Their other uses have 

 been already discussed. (Compare 

 Section A and Section v., 2.) 



2. On the approach of an enemy, 

 however, the crayfish takes to flight 

 as rapidly as possible. In escaping, 

 it swims off in jerks by executing 

 violent strokes with its abdomen 

 These strokes, being directed for- 

 wards, impel the crayfish backwards. 

 (If we stand at the prow of a boat, 

 and with the oar force the water away from the boat, the latter is 

 driven backwards. What is the behaviour of the crayfish when taken 

 out of the water ? Why can it not capture a fish or frog swimming ?) 

 Thus the abdomen forms a very important organ of locomotion : 



(a) For this reason, also, almost the whole of its interior is occupied 



Transverse Section through Thorax 

 of Crayfish (somewhat Diagrammatic) 



Heart ; D. , intestine ; Ns., the two nerve 

 cords cut across; K., branchise ; Kd., 

 branchial cover (branchiostegite) ; Kh. , 

 branchial chamber ; M. , muscles of legs. 



H., 



