NERVOUS SYSTEM AND BEHAVIOUR 435 



the single giant axon forms the final common motor path for all the longi- 

 tudinal muscles involved in the withdrawal reflex, and can be excited at all 

 levels in the animal. The giant axons oiProtula (Serpulidae) are connected 

 with a patch of sensory epithelium in the head, and are normally stimulated 

 in this region. They form a co-ordinating system lying penultimate to the 

 effectors. In Neanthes the single medial giant is fired by sensory stimuli in 

 anterior segments, the two small medial giants by stimuli in the posterior 

 three-quarters and the lateral giants by stronger stimuli at all levels of the 

 body. Transverse septa (synapses) occur in the lateral giant fibres of 

 Neanthes, but the fibres are not physiologically polarized and can transmit 

 in either direction (21, 24, 73, 74, 106a). 



Crustacea. Among Crustacea, giant-fibre systems occur in copepods, 

 stomatopods and decapods. Giant axons have been studied most intensively 

 in the freshwater crayfish, but comparable systems are found in lobsters 

 (Homarus), shrimps and prawns (Palaemonetes, Palaemon, etc.). Paired 



Motor ■ ^^^ Median 



giant- fibre ^^ ^^ ^^ 2><^ giant- fibre 



Lateral Median 



giant- fibre fused axon 



Fig. 10.12. Transverse Section through the Dorsal Part of an Abdominal 

 Ganglion of the Prawn Palaemon serratus, Showing Synapses of the Medial 

 and Lateral Giant- Fibres with the Motor Fibres (after Holmes (52).) 



median giant fibres originate in the brain and extend throughout the nerve 

 cord; besides these, a pair of lateral giants is found in thoracic and abdomi- 

 nal segments. The lateral giants are divided into segmental units by oblique 

 partitions or synapses, and the median giants of Palaemon show incomplete 

 septa. It appears that both lateral and medial giant fibres are syncytial 

 structures, connected with many nerve cells throughout their lengths. Fine 

 processes from lateral and medial giant axons form synapses with motor 

 neurones in each segment; from the motor neurones efferent axons pro- 

 ceed into the peripheral nerves (Fig. 10.12). The giant-axon system of 

 decapods is concerned with mediating an escape response: excitation of 

 this system causes the antennae to be drawn together and extended, and 

 the abdominal flexors to contract; the tail gives a sharp flip, and the animal 

 darts backwards through the water (20, 52, 113). 



Cephalopods. Giant axons of cephalopods (squid and cuttlefish) arise 

 from two first-order giant cells in the brain (posterior region of the pedal 

 ganglion) (Figs. 10.13, 10.18). The axons of these cells cross and fuse, and 

 in their further course form synapses with processes of second-order giant 

 neurones in the pallio-visceral ganglion. Second-order giant fibres pass 



