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CHAPTER V 



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WTH AND LIFE HISTORIES 



A l l the typical Crustacea grow in a series of steps. The presence 

 of a hard outer covering prevents any increase in size, except im- 

 mediately after moulting, when the new skeleton is still soft and 

 extensible. The life of most Crustacea is thus divided into intervals 

 between moults. These intervals may be further subdivided accord- 

 ing to the condition of the skeleton at the various stages. Carlisle 

 and Dohrn have produced a classification based upon their studies 

 of decapods, but essentially the same stages can be recognised in 

 other Crustacea, although the references to calcification do not 

 always apply. 



Stage i. Proecdysis or premoult — calcium is removed from the 

 exoskeleton, and the calcium content of the blood increases. 

 A new skeleton is laid down beneath the old one. New setae 

 can be seen forming. 



Stage 2. Ecdysis or moult — the actual shedding of the old exo- 

 skeleton; the animal swells by taking up water. 



Stage 3. Metecdysis or postmoult — the exoskeleton is being 

 hardened and calcified. Crabs and crayfish do not feed during 

 this stage, and do not expose themselves unnecessarily. 



Stage 4. Intermoidt — the animal is in normal condition, with a 

 hard exoskeleton. This stage varies in duration. When there 

 is a long interval between a metecdysis and the next proec- 

 dysis the term anecdysis is applied to the intermoult, but 

 when the metecdysis passes almost immediately into a proec- 

 dysis the term diecdysis is applied. Some Crustacea, after 

 passing through several moults, reach a stage where they do 

 not moult again; this can be called a terminal anecdysis. 



The mechanical process of moulting is often hazardous. It has 

 long been recognised by people who have kept Crustacea in aquaria 

 that difficulties during moulting, and failure to complete a moult 

 give rise to a large proportion of the deaths which occur. 



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