BY .TAS. P. HILL. 11 



heart bladder to form the proljoscis canal which opens on the 

 right side. 



With regard to the ])ehaviour of the dorsal proboscis pockets, 

 Pt. australiensis appears to Vje the most variable of all the 

 Enteropneusts hitherto descril^ed. 



N to chord (Eichel-darm of Spengel) : In shape the notochord 

 of this species essentially resembles that of Pt. minuta. Imme- 

 diately anterior to the point of opening of the lumen of the 

 notochord into the mouth or throat cavity, its narrow neck portion 

 is dorso-ventrally compressed, wdth a convex dorsal wall and a 

 concave thin ^^entral wall composed of a single layer of low 

 columnar cells resting on the proboscis skeleton (fig. 16, div.). 

 Anteriorly in the region of the proboscis neck the neck portion of 

 the notochord is not so much dorso-ventrally compressed, but 

 somewhat higher and with a more or less triangular lumen. The 

 dorsal wall of the neck portion of the notochord is very much 

 thicker than the ventral and is composed of long narrow epithelial 

 cells which radiate outwards from the lumen and have central 

 generally narrow rod-like nuclei. Between these elongated cells 

 there occur numerous clear oval bladders which Spengel well 

 regards as the secretory holders of gland cells. Some appear quite 

 empty, others again show a network in their interior similar to 

 that in the epidermal mucous glands and which stains in the same 

 diffuse manner. They thus conform, as Spengel has shown, to the 

 structure of the "goblet cells;" on the ventral side where the wall 

 is composed of a single layer of columnar cells, these gland cells 

 are entirely absent. The neck portion of the notochord is thus 

 distinctly epithelial in character. 



Anteriorly the dorsal wall increases considerably in thickness, 

 while the cells of the ventral wall lose their distinctly columnar 

 quality. They become longer and narrower, gland cells appear 

 between them, and they finally pass over into the chorda-like 

 tissue forming the wall of the ventral l:)lind sac of the notochord. 

 The lumen of the notochord extends obliquely downwards into 

 the venti-al blind sac (fig. 14), then in this the lumen extends 



\^ LIBRARY 



