BALANOGLOSSUS. 245 



longitudinal nerve-cords. In the region of the collar the 

 dorsal nerve-cord becomes entirely separated from the 

 ectoderm, and this portion of it contains, at least in young 

 individuals, a central canal which, from its origin and 

 relations, was shown by BATESON, and more recently by 

 MORGAN, to be homologous with the central canal of the 

 vertebrate spinal cord. Anteriorly the dorsal nerve-cord 

 becomes continuous with a specially dense tract of the 

 general nerve-plexus at the inner posterior surface of the 



ccm 



Fig. 114. Diagram of the organisation of Balanoglossus, from the left side. 

 (From a drawing kindly lent by Professor T. H. MORGAN.) 



al. Alimentary canal. Ac 1 . Ccelom of proboscis (anterior or prasoral body- 

 cavity). 6c' 2 . Coelom of collar. bc z . Coelom of trunk, b.v. Blood-vessel, proceed- 

 ing from the so-called heart (which lies at base of proboscis above the noto- 

 chord) to the ventral blood-vessel, ch. Notochord. com. Commissure, between 

 dorsal and ventral nerve-cords, dn. Dorsal nerve-cord, separated from the integu- 

 ment in the collar-region. d.b.v. Dorsal blood-vessel, gl. Proboscis-gland; 

 modified coelomic epithelium surrounding heart and front end of notochord. 

 m. Mouth, f.v. Pulsating vesicle, lying inside the " heart." v.b.v. Ventral blood- 

 vessel, v.n. Ventral nerve-cord. 



proboscis (Fig. 1 14). This proboscidian plexus thins out 

 somewhat towards the anterior extremity, but nevertheless 

 forms a complete nerve-sheath for the proboscis and indi- 

 cates the sensitive character of the latter (Fig. 115). 



The ventral nerve-cord does not extend into the region 

 of the collar, but from the point where the collar joins on 

 to the trunk the ventral cord is connected with the dorsal 

 nerve-cord by a commissure-like thickening of the integu- 

 mentary plexus, which passes in the skin on each side 

 round the hinder end of the collar-region (Fig. 114). 



