3o8 THE STARFISH less, xxvi 



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 (Fig. 76 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. The presence 

 of this structure disturbs the radial symmetry of the starfish 

 and gives rise to a bilateral symmetry, since the animal can 

 be divided into two truly equal halves by a single plane 

 only, viz.^ the plane passing through the middle of the 

 madreporite and of the arm opposite to it. 



The body, though flexible, is tolerably firm and resistant, 

 owing to the fact that immediately beneath the soft, slimy 

 skin there is a layer of little irregular calcareous bodies, the 

 ossicles (Fig. 76, os), forming a kind of scale armour. Many 

 of them give attachment to spines, and between them are 

 minute apertures, the dermal pores, through which, during 

 the life of the animal, are protruded delicate, glove-finger- 

 like processes, the dermal gills or respiratory cceca {Resp. 

 ca). Both on the dorsal and the ventral surfaces are found 

 curious and characteristic organs called pedicellarice {Fed). 

 These are minute forceps-like structures, consisting of a 

 basal piece or stalk and of two jaws, each supported by a 

 calcareous plate : the jaws are worked by muscles, and 

 apparently serve to remove faecal matter, foreign bodies, &c., 

 from the surface of the animal. 



The tube-feet, already referred to, are arranged symme- 

 trically on either side of each ambulacral groove. At the 

 extremity of the groove is a single structure (/) like a tube- 

 foot without the terminal sucker : it is called the tentacle, 



