EFFECTOR MECHANISMS 



379 



higher magnification provided by the electron microscope they can be 

 resolved into finer myofilaments. Some muscles have fine myofibrils 

 distributed uniformly throughout the sarcoplasm (e.g. adductor of My tilus, 

 spine muscles of sea urchin); some have a few large fibrillae distributed 

 around the periphery (e.g. muscles of Beroe, wing muscles of pteropods, 

 etc.); while still others contain large peripheral and fine central fibrils. The 

 relative amount of sarcoplasm shows much variation in different muscles. 

 Particularly striking are the large skeletal fibres of crustaceans, in which 



5 , 10 



Time (min) 







5 .10 



Time (min) 



Fig. 9.6. Effect of Extension on the Body Wall of Callicictis 

 Curves of extension x time, under two different loads (50 and 10 g) of intact {left) 

 and scraped {right) body wall. Note similarity of the viscous-elastic response of both 

 mesogloea + muscle and mesogloea only. (From Chapman (28).) 



the myofibrils are arranged in discrete columns, separated from one another 

 and from the sarcolemma by thick sarcoplasmal strata. 



Striped fibres, found in many phyla, contain myofibrils with cross- 

 striations aligned across the width of the fibre (Fig. 9.7). The stripes consist 

 of alternating anisotropic and isotropic bands, which tend to be spaced 

 more closely in fast than in slow muscles, e.g. in crustaceans and insects. 

 Usually the stripes lie transversely to the long axis of the fibre, but there 

 are some muscles in which the striations are spirally arranged (e.g. 

 adductors of some lamellibranchs). 



Smooth muscles, widespread in invertebrates and vertebrates, are 

 usually slower than striped muscles. Some invertebrate muscles contain 

 smooth and striped components, e.g. adductors of certain lamellibranchs; 

 the heart of Ciona contains fibres each of which is striped on one side 

 but plain on the other. Bozler associates large peripheral myofibrils with 

 fast tetanic contractions, and fine centrally dispersed myofibrils with tonus. 

 Muscles with fibres containing both kinds of fibrillae give both fast 

 (phasic) and slow (tonic) contractions. These correlations are yet to be 

 reconciled with recent physiological studies of slow and fast responses 

 produced by one muscle, of apparently uniform structure (cf. anemones, 

 sipunculoids, crustaceans) (21, 22, 37). 



Unusual kinds of muscle fibres have been described in coelenterates, 

 polychaetes and nematodes. Endodermal muscles of sea anemones are 

 made up of musculo-epithelial cells, each of which consists of an epi- 



