SKELETONS, SHELTERS AND SPECIAL DEFENCES 645 



Growth is normally associated with moult in crustaceans. Several 

 investigators have found that eye-stalkless specimens become larger than 

 normal. This is ascribed to the induction of extra moults and to an exces- 

 sive increase in volume at the time of moulting. The latter results from 

 derangement of water metabolism (117). It has long been known that female 

 crustaceans, carrying eggs, do not moult in spring as the males do, but 

 postpone their moult until after the young are liberated {Crangon and other 

 species). Egg-bearing female Cambarus can be induced to moult by removal 

 of the eye-stalks. The normal postponement of moult in the breeding 

 female, so essential to the survival of the species, is thus a function of the 

 eye-stalk neurosecretory organs. 



Moulting is attended by a series of complex metabolic changes, the 

 interrelations of which still await clear definition. Similarly, eye-stalk 

 ablation produces diverse alterations in physiological state which vary 

 with species, sex and stage of the moulting cycle. 



In the premoult period (stage D, Fig. 15.2) glycogen stores in the epi- 

 dermis and hepatopancreas reach a maximum; they fall to a minimum 

 after ecdysis during completion of the new cuticle (stage C). Calcium is 

 mobilized from stores in the digestive gland and elsewhere during the 

 period when the exoskeleton is being mineralized. Removal of the eye- 

 stalks produces no change in total glycogen, but does raise the glycogen 

 content of the epidermis. Other changes following loss of the eye-stalks are 

 hypoglycaemia, increase in calcium content of the digestive gland {Hemi- 

 grapsus, Panulirus) and loss of ability to regulate respiration (Gecarcinus). 

 It has been suggested that the eye-stalk principle or principles restrain inter- 

 mediary metabolism, especially those processes connected with preparation 

 for moulting (deposition of glycogen in the epidermis, deposition of 

 calcium in the hepatopancreas) (7, 32, 49, 66, 88, 115, 116). 



Evidence has also been adduced for a moult-accelerating principle in 

 decapods (prawns Lysmata, Palaemon). Extracts of the eye-stalks of fast- 

 moulting (summer) female prawns accelerate the rate of moulting, while 

 comparable extracts from eye-stalks of males or slow-moulting (winter) 

 females are less effective. It is suggested that the moulting cycle is initiated 

 by a fall in level of the moult-inhibiting hormone, but the moult, once 

 begun, is controlled by a moult-accelerating principle. This principle is 

 produced by neurosecretory cells in brain, thoracic ganglia, Y-organs and 

 eye-stalks (16, 18a, 19, 66a). 



Xiphosurans. The cuticle of Limulus is structurally similar to that of 

 decapod crustaceans. It contains very little inorganic matter (1% dry 

 weight). Organic matter consists of 25 % chitin, 75 % scleroprotein. The 

 latter is hardened by quinone tanning (10, 71). 



Mollusca. Several kinds of hard skeletons are found in the different 

 classes of molluscs. These are transverse articulating shell plates (Amphi- 

 neura); a single shell, often coiled (Gastropoda); a pair of hinged shell 

 valves (Lamellibranchia); tubular shell open at both ends (Scaphopoda) ; 

 and chambered external or reduced internal shells in cephalopods. The 



