788 Comparative Animal Physiology 



Among Crustacea giant fiber systems have been described in the cray- 

 fish, the lobster, and the prawns Palaemonetes and Leander. In the cray- 

 fish"^ there are three kinds of giant fiber: median, lateral, and motor. The 

 paired median giant fibers start in the brain (at least in the crayfish), de- 

 cussate there, and pass to the ventral nerve cord. The lateral fibers occur in 

 thoracic and abdominal ganglia and consist of segmental units which ap- 

 pear to overlap in long side-by-side contacts; they presumably are made up 

 of processes from ganglion cells in each segment. Branches are given off 

 from the laterals and medians to motor giant fibers in each segment. The 

 motor fibers have their cell bodies in one segment, decussate, and pass out 

 in the posterior nerve of that segment. ---'^ Stimulation of the giant fiber sys- 

 tem elicits quick flipping of the abdomen^^^ and causes extension and 

 drawing together of antennae.^^-' ^^^ The median and lateral systems inde- 

 pendently activate the same motor fibers. The median and lateral giant fi- 

 bers of the crayfish conduct equally well in both directions, but transmission 

 to the motor giant fibers is polarized. Crossing occurs from one lateral giant 

 to the other in 0.5 msec, but there is no crossing between the two median 

 fibers. There is no appreciable delay (less than 0.1 msec.) at the segmental 

 junctions of the lateral giants. Summation of the motor response occurs if 

 the faster median fibers are stimulated before the laterals. Probably the me- 

 dian giants are normally activated only in the brain, and the laterals at any 

 ganglion behind the fourth thoracic. 



In the prawn, Leander, -^^ two types of synapse exist: oblique regions of 

 apposition between the lateral fibers in each segment, and typical boutons 

 at the points of contact of processes from both median and lateral with the 

 motor fibers. The median giant fibers show incomplete septa. Transec- 

 tion of the cord at various points failed to produce degeneration of the med- 

 ian or lateral giant fibers. In Leander the motor giant fibers do not show 

 myelination from the cell body to their synapse but are myelinated from that 

 point out to the muscles. 



The cephalopod molluscs have a giant fiber system which is intermediate 

 between the unicellular type and the segmentally fused type. The cephalo- 

 pod system has been studied by Young.'^*^- *'*^' '*^^ In the decapods (squids) 

 a pair of giant cells lie at the posterior end of the pedal ganglion portion 

 of the brain; their processes fuse and cross, then synapse in the visceral 

 part of the brain with cell processes which run out in mantle nerves. These 

 synapse with a third set of giant fibers in the stellate or mantle ganglion. 

 From the stellate ganglion some dozen nerves pass out to the muscles of 

 the mantle; each nerve contains one giant fiber, which may be as much 

 as 800 jx in diameter, plus many 1 jx fibers. Each third order giant axon 

 arises by the fusion of processes from 300-1500 cells in the stellate gan- 

 glia. Each giant axon serves the circular muscles of a large area of the 

 mantle, and the largest fibers pass to the posterior part of the mantle so that 

 impulses arrive nearly simultaneously in the entire mantle.^*- Velocities of 

 conduction m fibers 30-718 /a in diameter range from 2.7 to 22.8 M./sec. 

 at 20° (Table 76). Stimulation of a giant fiber elicits a maximal contraction 

 at threshold, '*^^ and there is no increase in tension with increasing frequency 

 of stimulation, hence the one syncytial fiber with a large area of circular 

 muscle constitutes a motor unit. The contractile response forces water out 



