EFFECTOR MECHANISMS 377 



response involving the whole sponge. Conduction over short distances 

 (a centimetre or so) takes place through a spread of excitation from one 

 contractile cell to the next. Relaxation (opening) of the osculum takes 

 place more slowly than contraction (closure) and appears to depend on 

 elasticity of the tissue (95). 



MUSCLE 



Contractile elements occur in all phyla and include such diverse structures 

 as the contractile fibrils or myonemes of Protozoa, independent contractile 

 cells of sponges and muscle fibres of Metazoa. In free-swimming animals 

 muscular contractions provide the motive power for moving about to 

 gather food, evade enemies and engage in activities concerned with 

 propagation of the species. Sedentary animals move parts of the body 

 relative to one another in response to environmental stimuli and altered 

 internal conditions. The pattern, speed and duration of such movements 

 depend on the organization and functional characteristics of the animal's 

 muscles. Other muscles are concerned with ingestion and digestion of 

 foods, circulation of body fluids, respiration, excretion, secretion and 

 colour chanses. 



Muscles are vesicular or skeletal, depending on the organization of the 

 structures in which they operate; some muscles partake of both. Vesicular 

 muscles are arranged around fluid-filled cavities against which they exert 

 a compressive force. Such muscles occur in bands and sheets ; origins and 

 insertions are sometimes found in septa and mesenteries, but are often 

 ill-defined, each portion of the muscle being inserted into, and pulling 

 against, the next. Vesicular muscles contract against sacs of fluid that may 

 be termed fluid endoskeletons, and the contraction of one muscle, by 

 exerting pressure on the contained fluid, affects other parts of the cavity 

 and other muscle systems bordering it. Examples are body-wall muscles 

 of anemones, polychaetes; intestinal, bladder and cardiac muscles of 

 arthropods, molluscs and vertebrates. 



Skeletal muscles have their origins and insertions on the exo- and 

 endoskeleton. They usually operate as members of lever systems and 

 produce movement by shortening (isotonic contraction), or develop tension 

 with little or no change in length (isometric contraction). Examples are 

 phasic muscles of the limbs of vertebrates and arthropods, and adductor 

 muscles of lamellibranchs. There are instances of muscles motivating 

 hollow organs, with origins on skeletal structures and insertions on visceral 

 organs, e.g. external muscles of the swim-bladder in gadoid fishes, and 

 radial dilator muscles of the intestine of Crustacea. 



Muscles either produce movement (phasic activity) or maintain tension 

 (tonus). Most muscles are capable of both phasic and tonic activity at 

 different times, according to the functional conditions under which they 

 are called to operate. Anti-gravity muscles of vertebrates, for example, 

 maintain a basic level of tonus concerned with regulation of posture; 

 lamellibranch adductors are holding muscles but also close the valves. 



