390 



THE BIOLOGY OF MARINE ANIMALS 



stimulation. Another functional pattern is shown by the bender muscle of 

 crabs Dromidiopsis (Brachyura) and Dar claims (Anomura). This muscle 

 is innervated by two motor axons and is responsible for both bending and 

 rotation of the limb. Stimulation of the fast axon causes rotation in one 

 direction; stimulation of the slow axon, rotation in the opposite direction. 

 It is suggested that the three movements are produced by different groups 

 of fibres in the muscle. The bender fibres possess approximately equal 

 numbers of fast and slow endings; the headward rotator fibres have a 



Brachyura 



Anomura 



ur 



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7=: B 



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Palinura 



Astacura 



Fig. 9.13. Innervation Patterns of Distal Muscles in Thoracic 

 Limbs of Four Groups of Decapod Crustacea 



O, opener; C, closer; S, stretcher; B, bender; F, main flexor; A, accessory flexor; 

 E, extensor. Solid lines represent single motor axons; broken lines, inhibitory axons; 

 square brackets, distribution of axons between nerve bundles, the largest bundle being 

 on the right. In the Anomura the common inhibitor is variable in position and may 

 occur in the large nerve bundle. (From Wiersma and Ripley (127).) 



preponderance of fast endings; and the tailward rotator fibres a pre- 

 ponderance of slow endings (62, 120, 121, 127, 128). 



Lamellibranchs. The muscles of these animals have excited much 

 interest because of their strong maintained contractions (cf. p. 382). 

 Those which have been singled out for study include the adductors, closing 

 the valves, and the byssus retractor muscles. The adductor muscles exhibit 

 fast (phasic) and slow (tonic) contractions (Fig. 9.14). Most adductor 

 muscles contain both striated and plain fibres, mixed together (Lima), or 

 segregated into separate bundles (Pecten). Adductor muscles sometimes 

 show macroscopic colour differences into a grey component, striated and 

 responsible for phasic contraction; and a white component, containing 



