GROWTH AND LIFE HISTORIES 65 



enters terminal anecdysis after about ten moults from the onset of 

 maturity. The Y-organ of Carcinus does not degenerate, but when 

 it enters its terminal anecdysis the crab is prevented from moulting 

 by an excessive secretion of moult-inhibiting hormone by the X 

 organ-sinus gland complex. If the eyestalks of Carcinus in terminal 

 anecdysis -are r emov ed then the crab may moult, and larger 

 specimens than usual may be produced by this means. 



The hormonal system controlling moulting does not act 

 independently of the environment. Moulting may be inhibited by 

 unfavourable conditions of various kinds. Prolonged starvation is 

 an obvious example, but moulting of the crab Gecarcinus lateralis 

 is also inhibited by continuous light, by high temperatures, and 

 even by the absence of a suitable burrow. This last factor indicates 

 that the hormonal system may be influenced by physical contact 

 of the animal with its sheltering place. This is of some importance 

 to a crab because it is more vulnerable than usual after moulting, 

 and having a shelter will increase its chance of survival. 



The swelling which occurs when the old skeleton is cast is due 

 to the intake of water. This process is also under hormonal control. 

 When the eyestalks of the crab Carcinus are removed a day or so 

 before moulting the operated crabs swell to a greater size than 

 unoperated controls. 



The hormone which controls water intake appears to be different 

 from that which inhibits moulting. The Plymouth population of 

 Carcinus maenas only produces the moult-inhibiting hormone in its 

 eyestalks during the three coldest months of the year; at other 

 times brain secretions are responsible for moult inhibition. However, 

 removal of the eyestalks at any time of year results in an increased 

 swelling at a moult. 



Swelling after a moult is a comparatively rapid process; in the 

 crayfish the increase in size is completed in about six hours. In 

 smaller Crustacea the process is much more rapid; Daphnia com- 

 pletes its swelling within a minute. Such a rapid intake of water 

 must result in a dilution of the blood, and this has been found to 

 be so in a wide range of Crustacea. The concentration of the blood 

 is usually restored to normal within a short space of time. This 

 temporary dilution of the blood produces an odd effect in Daphnia, 

 where a disturbance of the heart rate for about five minutes after 

 moulting has been observed. This may be explained by the dilution 

 of certain ions, some of which, such as potassium, have important 

 effects on the rate and strength of the heart beat. 



