394 



THE BIOLOGY OF MARINE ANIMALS 



fibrils are composed largely of actomyosin, plus a small amount of tropo- 

 myosin. Myosins from widely disparate groups show much similarity in 

 chemical properties. An additional unit, paramyosin, has been identified 

 in molluscan smooth muscle. The proteins of the sarcoplasm are globular 

 molecules and consist largely of enzymes. 



The myosin filaments are restricted to the A bands, whereas the actin 

 filaments extend from the Z line through the / band into the A band. It is 

 believed that the actin filaments lie between the myosin filaments, with 

 which they form temporary cross-linkages. During contraction the actin 

 filaments are drawn past the myosin filaments into the A band. The 



Fig. 9.16. Contractions and Action Potentials from Proboscis 

 Retractor Muscles of Golfingia gouldii 



{Above) contractions, a, b, at 5/sec; c, d, at 20/sec; a, c at low intensity which elicited 

 spike only ; b, d at higher intensity which elicited spike and slow wave. (Below) a con- 

 traction and b action potentials at 9-6/sec at intensity eliciting both spike and slow wave; 

 c contraction and d action potential at 9-6/sec at intensity eliciting spike only. Time 

 trace, 01 sec. (Prosser et al. (103).) 



immediate energy for contraction is provided by the high-energy bonds 

 of phosphate groups (adenosine triphosphate, ATP, and inosine tri- 

 phosphate), which interact with actomyosin. 



Another phosphate donor, phosphagen, is peculiar to muscle, and is 

 concerned with maintenance of the level of ATP during muscle activity. 

 The phylogenetic distribution of phosphagens has been a matter of con- 

 siderable interest (Table 9.3). Phosphocreatine is the characteristic 

 phosphagen of vertebrates, but also occurs in protochordates (Amphioxus, 

 Balanoglossus) and echinoderms (ophiuroids and echinoids). In the 

 majority of invertebrates the phosphagen is phosphoarginine. In Balano- 

 glossus and certain echinoids both creatine phosphate and arginine 

 phosphate are present. 



In polychaetes and sipunculoids a new phosphagen has been reported 



