XII CRUSTACEA 163 



In the basal joint of each first .antenna there is a little 

 cavity from the walls of which spring hairs supplied with 

 nerves at their bases ; the cavity contains a slightly gelatinous 

 liquid in which are small solid particles, placed there apparently 

 by the prawn itself. This little organ, which is in free com- 

 munication through a small opening with the water outside, 

 is undoubtedly an organ of equilibiium, and probably an 

 auditory organ as well. The prawn is thought to have like- 

 wise a sense of smell located in certain peculiar hairs on both 

 pairs of antennae. 



Other Long-tailed Decapod Crustacea. 



The Shrimp (Crangon vulga/ris) is very common 



Shrimn '''^ ^'^'^ shores, and can be at once distinguished 



from the common prawn by its smaller size, by 



the form of its body, which is flattened from above rather 



than laterally, and by the absence of a prolonged rostrum, as 



also by the already mentioned fact that it does not turn 



a bright red when boiled. 



Lobsters and Crayfishes have very similar 



CravShes structure to that of shrimps and prawns, though 



they are, of course, much larger. 



In lobsters the first " walking legs," which are the largest, 



are nearly always unequal in size on the two sides, one 



forming smaller cutting nippers, whilst the other is larger 



for crushing food. These legs have apparently six instead of 



the usual seven joints, for the second and third joints are 



fused together. A curious characteristic of these, and of 



many Crustacea, is their power of casting off part of a limb if 



it is injured or seized by an enemy ; the end of the leg is 



voluntarily cast away, the breaking point being always 



across the centre of the second and third fused joints ; 



excessive bleeding at this point is prevented by a special 



membrane, which is pushed inwards at the point of rupture. 



All young lobsters and the adult males are said to moult 

 twice a year, but adult females only once ; it is a dangerous 

 process, and not infrequently causes the death of the 

 creature, either owing to some injury whilst it is taking 

 place, or to its defenceless condition if some enemy should 

 then find it. 



Lobsters breed only once in two years, usually in July or 



