OUR KNOWLEDGE OF NEW ZEALAND HOLOTHURIANS. 



117 



The intestine appears to be uniform in structure throughout its whole 

 length and differs from the pharynx only in the following details : — 



(i.) The epithelium is much more glandular and its surface is much more 

 irregular than in the pharynx. 



(ii.) There appear to be many more wandering-cells both in the epithelium 

 and in the inner connective-tissue layer. 



Calcareous Ring. — The calcareous ring is composed of ten simple, somewhat 

 dumbbell-shaped pieces, five radial and five inter-radial. These pieces are all 

 very similar to one another, and the difference between radials and inter- 

 radials can only be recognized by their position with regard to other structures, 

 especially by the attachment of the longitudinal muscles to the radials. The 

 natural clefts between the pieces are overlapped and almost concealed by the 

 diverticula of the tentacular canals (fig. 17), and at first sight the junctions 

 appear to be in the constricted portions of the ring between the tentacular 

 canals. This appearance, how^ever, is deceptive, and the true relation of the 

 constituent parts of the ring to the radial muscles and tentacular canals is 

 shown diagrammatically in the accompanying text-figure (fig. C). 



Fiff. C. 



^nd /'h ^ 



m.t. 



Diagrammatic transverse section through the calcareous ring of Rhabdomolgus novcB- 

 zealandicB. — f/ = diverticula of tentacular canals; i.r. = inter-radial ; l.m. = longi- 

 tudinal muscle-hand ; m.d. = muscular wall of diverticulum ; ??ii. = muscular 

 wall of tentacular canal ; r = radial ; t.c. = tentacular canals ; v. = valve. 



Water-vascular System. — The circular canal is a thin-walled tube which 

 encircles the pharynx just behind the calcareous ring (fig. 17, a.c). Its wall 

 is only 12 fx thick, but yet the following layers can be distinguished : — 

 (i.) An outer layer of peritoneal cells ; (ii.) a connective-tissue layer ; 

 (iii.) a clear zone which does not appear to contain either cells or nuclei : 

 (iv.) a thin layer of circular muscle-fibres ; (v.) a lining endothehum 

 composed of cells, probably ciliated, resembling peritoneal cells. 



The circular canal gives off a single madreporic canal (m.) dorsally, a single 

 Polian vesicle {p.) ventrally, and in addition ten adradially situated tentacular 

 canals {t.c.) to the tentacles. 



