378 THE BIOLOGY OF MARINE ANIMALS 



Often muscles occur in pairs, producing reciprocal movements: thus the 

 longitudinal muscle of the annelid body wall is opposed by a circular 

 muscle layer, the former shortening, the latter lengthening a segment. 

 Phasic skeletal muscles occur in antagonistic pairs, the two members of a 

 pair producing movement in opposite directions. In some instances, how- 

 ever, the muscle works against a non-muscular mechanical antagonist, 

 e.g. the hinge ligament of lamellibranchs and the frontal surface or 

 compensation sac of cheilostomatous polyzoa. 



Muscles are specialized for movement at various speeds, and there is all 

 manner of gradation in contraction speeds, from slow holding muscles 

 to phasic muscles producing rapid locomotory movements. 



Most muscles are subject to nervous regulation. Some visceral muscles, 

 notably cardiac muscle, show automatic activity which is varied under 

 nervous influence, e.g. molluscan heart (Chapter 3). Gut muscles often 

 contain an intrinsic nervous plexus concerned with co-ordination of 

 activity and transmission of excitation. Locomotory muscles are excited 

 by the nervous system (either nerve-net or central nervous system), and 

 are the principal agents for executing spontaneous and reflex activities. 

 Speed and strength of locomotory responses are controlled in part cen- 

 trally, in part peripherally. Regulatory mechanisms involve: gradation 

 in number of active muscle units ; variation in number and frequency of 

 nervous impulses ; interaction of peripheral inhibitory and excitatory fibres ; 

 and stimulation by nerve fibres producing different degrees of excitation. 

 Excitation mechanisms of various kinds have now been investigated in 

 several phyletic groups (55a). 



Histology of Muscle 



The fibres or muscle cells responsible for contraction are embedded in 

 connective tissue, which imparts some degree of viscous resistance to 

 contraction. The proportion of connective tissue is sometimes very high, 

 e.g. in the longitudinal retractors of holothurians (Throne). The body wall 

 of sea anemones contains a relatively small amount of muscle associated 

 with a thick layer of mesogloea, and it is the latter which is responsible 

 for the viscous elastic properties of the wall (Fig. 9.6). 



There is a wide range in length of fibres from different muscles. Some 

 extremes are plain muscle fibres in vertebrate blood vessels, 15-20 ju long; 

 and plain muscle fibres in the anterior byssus retractor of Mytilus, 4-6 cm 

 long. Short plain muscle fibres are often uninucleate (coelenterates, 

 sipunculoids, holothurians, annelids, etc.); long fibres, plain and striped, 

 may contain many nuclei (crustacea, lamellibranchs, vertebrates, etc.). 

 Some cardiac muscles are syncytial, i.e. the fibres branch and anastomose; 

 consequently, excitation passes unhindered throughout the body of the 

 muscle (e.g. vertebrate heart) (28, 47, 89, 90, 104). 



Each fibre is bounded by a membrane or sarcolemma and contains a 

 nucleus (or nuclei), contractile myofibrils and sarcoplasm. Myofibrils 

 can usually be distinguished with the light microscope, and under the 



