BEHAVIOUR OF BALANOGLOSSUS 



55 



lines, and in the dorsal part of the collar region it is rolled up as a 

 hollow neural tube, open at both ends (Fig. 24). These unmistakable 

 resemblances not only to the uncentralized sub-epithelial plexus of 

 echinoderms but also to the hollow dorsal nerve-cord of vertebrates 

 are most instructive, showing the affinity of the groups and the origin 

 of the general plan of the vertebrate nervous system. There are no 

 organs of special sense, unless this is 

 the function of a patch of special cili- 

 ated cells on the collar. Receptor cells 

 all over the body send their processes 

 into the nerve plexus (Fig. 23), on the 

 primitive plan of neurosensory cells 

 found elsewhere in vertebrates only in 

 the olfactory epithelium and the retina. 

 The plexus is remarkable in receiving 

 fibres from the outer ciliated epithelial 

 cells, which thus represent the epen- 

 dyma, the earliest form of neuroglia 

 (Fig. 23). Nothing is known of the 

 organization of pathways or of the 

 connexions with the muscles. The 

 collar nerve-cord contains giant nerve- 

 cells whose axons proceed backwards 

 to the trunk and forward to the pro- 

 boscis (Fig. 28). They are probably 

 responsible for rapid contractions 

 (Knight-Jones, 1951). 



Bullock has investigated the beha- 

 viour of the animals and found only 



one clear-cut reflex, namely, a contraction of the longitudinal muscles 

 in response to tactile stimulation. Isolated pieces of the body are able 

 to show reflex responses, moving away from light or tactile stimuli. 

 Such local actions are an interesting sign of the uncentralized nature 

 of the nervous system, and similar actions are found in echinoderms. 

 A further sign of lack of special conducting pathways is that stimula- 

 tion of flaps of body wall partly severed from the rest produces 

 generalized contraction, proving that conduction can occur in all 

 directions. The dorsal and ventral nerve-cords do, however, act as 

 quick conduction pathways, and contraction of the trunk following 

 stimulation of the proboscis is delayed or absent if one, and especially 

 if both, cords have been cut. 



Fig. 28. Diagram of certain tracts in 

 the nervous system of Balanoglossus . 



com. circular connective; col.coel. collar 

 coelom; col.n.c. collar nerve-cord; ep.pl. 

 nerve plexus in epidermis of trunk; gp. 

 gill pore; tr. coel. trunk coelom; tr.n.c. 

 trunk nerve-cord. (From Bullock, J. 

 Comp. Neurol., vol. 80, by permission.) 



