382 



THE BIOLOGY OF MARINE ANIMALS 



economy of effort. Some muscles develop tension rapidly, e.g. the striped 

 mantle muscles of the squid, concerned with rapid expulsion of water from 

 the mantle cavity, with a contraction time (to peak tension) of 60 msec, 

 and relaxation time of some 200 msec. Others, the tonic muscles, contract 

 slowly but maintain strong tensions for long periods, e.g. the anterior 

 byssal retractor of Mytilus, with a contraction time (to peak tension) of 

 5 sec and relaxation time of some 13 sec. There are many smooth muscles 

 that are homogeneous and lack morphological differentiation, and yet 

 give both fast and slow contractions. An instance is the mesenteric re- 

 tractor ofMetridium which gives a fast facilitated response to fast stimulation, 



40 45 50 



Muscle Length (mm) 



Fig. 9.8. Maximum Tension Developed at Different Lengths in Isometric 

 Contraction of Dogfish Jaw Muscle {Scyliorhinus canicula) 



Temp. C C; stimulation frequency 5/sec; muscle length 37 mm, weight 600 mg. 

 (From Abbott and Aubert (1).) 



and a smooth delayed contraction to low-frequency stimulation (contrac- 

 tion times of 1 sec and 15 sec respectively) (3, 16, 17, 47, 101, 107a, 

 113). 



The tension developed by a muscle during contraction is influenced by 

 its initial length, as illustrated in Fig. 9.8. With increase in initial length 

 there is an increase in tension until some optimal value is reached, beyond 

 which point the tension developed begins to fall. As a rule, higher tensions 

 are developed by the slow muscles of invertebrates. The plain adductors of 

 lamellibranchs are notable for long-sustained contractions against strong 

 pulling forces. The data collected in Table 9.1, although difficult to com- 

 pare, give some idea of estimates of muscular tension which have been 

 made. Estimates of tensions needed to force open the valves of lamelli- 

 branchs, by overcoming the pull of the adductor muscles, are shown in 

 Table 9.2 (1, 2, 3, 14, 48, 65, 94, 118). 



