BEHAVIOUR gi 



burrow is occupied, and as such it is usually respected, indicating 

 that the sound has been perceived by the exploring crab. 



Another set of sense organs is concerned with giving a crustacean 

 information about the disposition of its parts. It is always useful to 

 know what position one's abdomen is in, or where one's legs are 

 without having to look for them. This information is provided by 

 nerve cells which are situated on or supply processes to special 

 small strands which are stretched to varying extents when the main 

 muscles contract. 



PROPODITE 



APODEME TO WHICH EXTENSOR 

 ^MUSCLE IS ATTACHED 



SENSE ORGAN 



INT OF ATTACHMENT 

 IN DACTYLUS 



DACTYLUS 



SENSE ORGAN 

 NERVE CELLS 



APODEME OF FLEXOR MUSCLE 



TO SENSE ORGAN 



Fig. 41. Diagram of a sense organ near the end of a leg of the 

 shore crab Carcinus maenas. A shows the general arrangement. 

 B shows the details, including the nerve cells which perceive 

 changes in the length of the organ according to the position of 

 the dactylus. The apodemes are prolongations from the skeleton 

 of the dactylus which serve for the attachment of muscles. 

 (After Burke, 1954.) 



One such organ is found near the end of the legs of the shore 

 crab, Carcinus maenas (fig. 41). This is arranged so that when the 

 dactylus is flexed the organ is stretched and the nerve cells in the 

 propodite relay information to the central nervous system. Other 

 stretch receptors are found in the muscles which run from the 

 thorax to the abdomen in lobsters and prawns. These receptors are 

 modified muscles, often very thin, with a number of nerve cells 

 supplying them. 



Similar structures have been found at the bases of the legs in 



