Nervous Systems 787 



type consists of a syncytium in which processes from a number of nerve cells 

 fuse to form a single giant axon. Unicellular giant fibers are found in 

 nemerteans and cestode worms, in numerous polychaetes, in balanoglossids, 

 and in lower vertebrates. In the polychaete Halla,^^ for example, there are 

 groups of unipolar giant cells 30 - 150 yu, in diameter in each ganglion of 

 the first few segments; these give rise to fibers up to 40 fx. thick which may 

 run the length of the worm. Similar unicellular giant fibers have been de- 

 scribed in other worms. Typically, in balanoglossids there are about a dozen to 

 150 giant cells, usually in the collar nerve cord; these give rise to large (3-6 

 yu.) fibers which run back in the nerve cord and eventually dwindle in size 

 to merge with the ordinary (1 /t) fibers.'^^' ^^^ Nerve cells of the unicellular 

 type of giant fiber, Mauthner and Miiller cells, occur in the base of the 

 medulla as part of the vestibular reflex system of bony fishes, urodeles, and 

 tadpoles of anurans. These fibers, 22-43 /i, in diameter, conduct at 50-60 

 M./sec. at 10-15° C. (Table 76).i6» 



The multicellular or syncytial type of giant fiber system has been de- 

 scribed in annelids, crustaceans, and cephalopod molluscs. 



In the ventral nerve cord of the earthworm there are in each segment 

 several cells which send axons to the three dorsal giant fibers, which occur 

 as one median and two smaller lateral fibers. These giant fibers run the 

 full length of the ventral nerve cord and give off segmental branches to 

 motor neurones in the cord. The median giant fiber is connected in each seg- 

 ment to approximately two pairs of giant cells and each lateral giant fiber to 

 approximately one pair of giant cells.^"'^' The lateral giant fibers are also con- 

 nected with each other. In addition to the three dorsal giant fibers there 

 are two smaller ventral ones.^^^- ^^^ At each segment there is in the giant 

 fibers an oblique partition, the two sides of which stain differently.'^^' ^^^ 



Evidence from cutting the giant fibers, from regeneration, from blocking by 

 drugs, and from embryonic development indicates that in the earthworm the 

 giant fibers are responsible for quick end-to-end "startle" contractions.^^' *^® 

 Stough^^- cut single giant fibers and concluded that the median one conducts 

 posteriorly and the two lateral ones conduct anteriorly. Action potentials,^^^ 

 however, showed that conduction is equally good in eUher direction in each 

 fiber. Bullock"^® settled the disagreement by showing that the median fiber is 

 normally excited by sensory stimulation in the anterior forty segments, 

 whereas the lateral fibers are excited by sense organs behind that level. The 

 two lateral fibers and the median fiber form two separate conduction and ex- 

 citability systems-; cross connections between the lateral fibers were shown by 

 conduction which continued when right and left fibers were alternately 

 cut.^''° Usefulness of the giant fiber system, particularly in protective with- 

 drawal reactions, is indicated by the great difference in speed of conduc- 

 tion: conduction in the smaller fibers of the cord (including synapses) is 

 about 0.025 M./sec.,^^ whereas in the lateral giant fibers it is 7-12M./sec. 

 and in the median fiber it is 17-45 M./sec, depending on fiber size.'^*'- ^^* 

 Some polychaete worms have no giant nerve fibers (e.g., Afhrodita, Chae- 

 tof terns') J others have only one QArenicola, Pista), and still others have 

 several, as shown both by the action potentials in their nerve cords and "by 

 histological examination QNeanthes, Glycera).^^ The junctions in Neanthes 

 are complex and are polarized histologically but not physiologically. 



