436 



THE BIOLOGY OF MARINE ANIMALS 



out in posterior infundibular, visceral and pallial nerves, and those run- 

 ning in the latter make contact with a third set of giant fibres, arising in 

 the stellate ganglion. The third-order giant fibres are multicellular in 

 origin, each arising from some 300-1,500 cells (Fig. 10.14). The axons run 



Fig. 10.13. Diagram of the Giant-axon system of Loligo 



Nerve centres and nerves in outline; giant axons in solid colour or stipple. 1, 2, 3 

 first-, second- and third-order giant axons. (From Young, 1936.) 



m.c. 



s.$f. 



mm. 



tqf. v St.q. 



Fig. 10.14. Giant Axons of Cephalopods (a, Loligo, b, Sepia) 



A pre-synaptic (second-order) giant fibre is shown terminating on third-order giant 

 fibres. The latter have a multi-cellular origin and proceed to the mantle muscles, m.c, 

 mantle connective (pallial nerve); m.m., mantle muscle; s.g.f., second-order (pre- 

 ganglionic) giant fibre; st.g., stellate ganglion; st.n., stellar nerve; t.g.f., third-order 

 giant fibre. (From Young, 1936, 1938.) 



out in the stellar nerves to the mantle, each giant axon supplying a large 

 area of circular muscle in the mantle wall. Activation of the giant-axon 

 system elicits nearly simultaneous contraction of the entire circular muscu- 

 lature of the mantle ; the compressed water is forced out of the funnel and 

 drives the animal forward or backward, according to the direction in 

 which the funnel is pointed. 



