64 A BIOLOGY OF CRUSTACEA 



on the fractional pull on the tip of the seta as it is withdrawn from 

 the old seta. If there is not sufficient friction the seta is not extended, 

 if there is too much the animal is stuck and encumbered with at 

 least a part of its old skeleton. 



The process of moulting is controlled by hormones, which are 

 secreted by various glands. Some of these glands have already been 

 described in the chapter on colours (p. 58). If the eyestalks of a 

 crayfish are removed the animal moults earlier than it would 

 otherwise. Now, if sinus glands are implanted in the operated cray- 

 fish the moult is delayed. This is a clear indication that a moult- 

 inhibiting hormone is present in the sinus gland. There is also 

 evidence, from other experiments, that a moult-inhibiting hormone 

 may sometimes be produced by the brain. 



A moult-accelerating hormone has also been found, surprisingly 

 also in the eyestalks, but not in the sinus gland. In Lysmata this 

 hormone appears to be produced in the medulla terminalis X organ 

 (see fig. 29, p. 58). 



An interesting situation has been found in relation to the dual 

 control of moulting in the prawn Leander serratus. It was found 

 that removal of the eyestalks of specimens from Roscoff led to an 

 increase in the rate of moulting, whereas if the eyestalks were 

 removed from Plymouth specimens a decreased rate of moulting 

 was the result. It is clear that the hormone balance in the two 

 populations is different, though the nature of the difference is not 

 known. It may be due to a different rate of production of moult- 

 controlling hormones in other parts of the body. 



The Y-organ is also involved in the control of moulting. This 

 organ, which lies in the head, seems to produce a hormone which 

 is the actual stimulant for the onset of proecdysis, and the sugges- 

 tion has been made that the moult-accelerating and inhibiting 

 hormones act via their effects on the Y-organ. The role of the 

 Y-organ in relation to the terminal anecdysis of crabs has recently 

 been elucidated by Carlisle. There are two possibilities in explaining 

 the lack of any further moulting. One is that the Y-organ degener- 

 ates and no longer produces the moult-promoting hormone; the 

 other is that the Y-organ does not degenerate, but is inhibited by 

 excessive production of moult-inhibiting hormone. Both these 

 mechanisms have been shown to operate in different species of 

 crabs. Maia squinado does not moult once it has become sexually 

 mature, and at the same time its Y-organ degenerates. Carcinus 

 maenas continues moulting while sexually mature, but eventually 



