434 



THE BIOLOGY OF MARINE ANIMALS 



nized: in some animals the giant axons are unicellular neurones (poly- 

 chaetes Sigalion and Halla) ; in others they are multicellular and syncytial 

 (sabellids, cephalopods). It is quite common to find giant axons extending 

 long distances through the c.n.s. (polychaetes, crustaceans), or into peri- 

 pheral nerves (crustaceans, cephalopods). Brief descriptions of the giant- 

 axon systems of some representative species will illustrate various charac- 

 teristics and form a basis for interpreting their functions (73, 89, 101, 128). 

 Polychaetes. Polychaete families with well-developed giant-axon systems 

 are Nereidae, Arenicolidae and Sabellidae. In Neanthes virens there are 

 two lateral giant fibres and a single median fibre which extend throughout 

 the length of the nerve cord. Nerve cells require further study, but it 

 appears that the median fibre is connected with nerve cells anteriorly and 



Giant 



Longitudinal 

 muscle. 



Giant axon 

 motor branch 



Nerve 

 cell 



Fig. 10.11 Section through the Nerve Cord of Myxicola infundibulum 



The giant axon is joined to many nerve cells and gives off motor branches to the 

 longitudinal muscles and integrating branches to the neuropile. 



the lateral fibres with nerve cells at all levels. The lateral fibres make con- 

 tact in each segment with a pair of anastomosing neurones which extend 

 into the peripheral nerves. There is also a peculiar system of paired small 

 medial giants, which consist of a chain of simple units connected in series 

 by synapses (Fig. 10.2). A giant-fibre system is highly developed in sabel- 

 lids and serpulids, which possess one or two large axons extending 

 throughout the length of the nerve cord (Fig. 10.1 1). The giant axons arise 

 from nerve cells in the brain and connect with trophic nerve cells through- 

 out their length. In each segment they give off motor branches to the 

 longitudinal muscles. 



The intersegmental giant axons of tubicolous and burrowing polychaetes 

 are involved in the quick shortening or withdrawal reflex by which the 

 animal jerks back into its shelter when disturbed. Conduction velocities 

 are high when compared with neuropile transmission, up to 5 m/sec in 

 Neanthes and 20 m/sec in Myxicola (Table 10.2). Since the axons extend 

 through many segments and serve wide-spread muscles, an impulse arising 

 at any level can bring about synergic contraction of the latter. In Myxicola 



