92 ABIOLOGYOFCRUSTACEA 



several different decapods. In the lobster there is in addition a 

 second organ running close to the modified muscle, but not appar- 

 ently formed from muscle. Instead the second organ appears to 

 consist of a tube of elastic fibres with a row of nerve cells inside it. 

 Recent work by Alexandrowicz (1958) suggests that one organ may 

 be concerned with registering forward movements and the other 

 with registering backward movements of the leg. Other types of 

 nerve cells, the N-cells, have been found on the surfaces of ordinary 

 muscles in the thorax of lobsters and prawns; their function is 

 unknown, but they presumably gather some sort of information 

 about the state of the muscles. 



Crustacea also have a well-developed sense of touch. This may 

 seem surprising for a creature with a hard outer skeleton, but the 

 numerous setae over the surface of the body are supplied with 

 nerves and any deflection is relayed to the central nervous system. 



A sense of smell, which is the ability to perceive chemical sub- 

 stances, is also present. Specially modified setae, with very thin 

 walls to allow diffusion of substances to the sense cells, are found 

 particularly on the antennules, antennae and mouthparts. Perhaps 

 the setae on the mouthparts are more concerned with what we call 

 taste, but it is difficult to distinguish between taste and smell in a 

 creature like a crab; it is better to use the general term chemo recep- 

 tion, which covers both. Experiments have shown that crabs can 

 respond to incredibly low concentrations of substances like vanilla 

 and acetic acid. 



The Crustacea are thus well aware of several important features 

 in their environment, and have a knowledge of their position with 

 respect to light and gravity, and of the disposition of their parts 

 when they adopt any particular posture. 



A particular posture is often adopted for a particular purpose. 

 We have already seen (p. 51) how the land crab Sesarma meinerti 

 adopts a pugnacious attitude to back up its warning coloration. 

 Another posture is adopted by the spider crab Hyas coarctatus. 

 This species hides among seaweeds, but its claws, each of which has 

 a conspicuous pink patch, are waved gently in the water to attract 

 small fishes. When these come near enough they are captured and 

 eaten. 



Posturing of a different type is seen in the courtship displays of 

 J fiddler crabs. These crabs belong to the large genus Uca which con- 

 tains many species with brightly coloured males. The males have 

 one chela enormously enlarged in comparison with the other (fig. 



