GROWTH AND LIFE HISTORIES 63 



The old skeleton is split along definite pre-formed lines. In the 

 lobster, for instance, the skeleton splits between the carapace and 

 the abdomen, and the body is withdrawn through the hole leaving 

 the old skeleton practically intact. In the isopods the skeleton is cast 

 in two parts, and the front portion may be cast several days after 

 the hind portion. Some Cladocera, such as Ilyocryptus and 

 Monospilus, do not completely cast the carapace at each moult, so 

 that they accumulate a series of carapaces on their back. One other 

 cladoceran, Alonopsis elongata, has a similar habit which appears 

 to be of survival value. Chaetogaster diaphanus is a worm which 

 feeds on small Crustacea; in a survey of its feeding habits in 

 Windermere it was found that Alonopsis elongata occurred many 

 fewer times in the worm's gut than one would expect from its 

 abundance in the immediate vicinity of the worm. This could be 

 explained by the fact that A. elongata carries one or two old cara- 

 paces, out of which it can escape if trapped. 



Long, delicate setae, such as those on the antennae of Daphnia, 

 present certain problems during ecdysis. These problems are over- 

 come by the setae developing in pockets formed by their own bases. 

 During moulting these pockets are extroverted and the setae 

 extended (fig. 30). The extension of these setae illustrates the delicate 

 balance between success and failure in moulting. Extension depends 



Fig. 30. Diagram to show the formation of new setae 

 during moulting. The new seta lies partly within the old 

 seta and partly tucked back in the newly formed cuticle. 



