LESS, xxvn TUBE-FEET 311 



The circular blood-vessel (C. B. V} surrounds the gullet and gives 

 off radial vessels (Rad. B. V} to the arms and an inter-radial plexus 

 connected with a pentagonal ring round the intestine. 



The circular ambulacral vessel (C. Amb. V} gives off radial vessels 

 (J\ad. Amb. V) to the arms connected with the ampullae (Amp) and 

 tube-feet (T. F) : it is also connected with the stone-canal (St. C], which 

 opens externally by the madreporite (Mdpr}. 



The nerve-ring (Nv. R) gives off radial nerves (Rad. Nv} to the 

 arms. 



The ovary (Ovy) is inter-radial, and opens by a dorsal oviduct (Ovtf). 



In the centre of the disc on the ventral surface is the large 

 mouth (Fig. 76, A, Mth\ and from it radiate five grooves, 

 one along the ventral surface of each arm (A and B). In the 

 living animal numerous delicate semi-transparent cylinders, 

 the tube -feet (T. F}, are protruded from these grooves; they 

 are very extensible and each ends in a sucker. It is by 

 moving these structures in various directions, protruding 

 some and withdrawing others, that the starfish is able to 

 move along either a horizontal or a vertical surface, and 

 even to turn itself over when placed with the ventral side 

 upwards. 



Near the middle of the disc, on the dorsal surface, is the 

 very minute anus (A, An) ; it is situated on a line drawn 

 from the centre of the disc to the re-entering angle between 

 two of the rays, and is therefore said to be inter-radial in 

 position. Near the anus, and also inter-radially situated, is 

 a circular calcareous plate, the madreporite (Mdpr\ per- 

 forated by numerous microscopic apertures. Innumerable 

 other calcareous plates, or ossicles (os\ are embedded in 

 the body-wall, and constitute a skeleton, to which the firm 

 and resistant character of the starfish is due. 



Sections show that there is a well-marked ccelome, separ- 

 ating the body-wall from the enteric canal and containing 

 the gonads, blood-vessels, &c. The body-wall consists ex- 

 ternally of a very thin cuticle, then of a layer of deric 



