ASTERIAS. 



157 



middle of the upper surface is a minute aperture, the 

 anus^ and placed eccentrically in one inter-radius is a round, 

 slightly convex, perforated plate called the madreporite. 

 The Hne drawn through anus and madreporite and 

 continued on either side gives the direction of the perpen- 

 dicular plane of symmetry. The whole upper surface of the 

 body is covered with a scattered mass of ossicles^ calcareous 

 nodules which can be seen and felt through the extremely 

 thin and delicate ectoderm. 



Fig. 90. — Transverse Section of the Arm of 

 AsTERiAS Rubens. (Diagrammatic.) 



Epigastric Coelom. 



Mesentery. 

 Pyloric Gland. 



Gonad. „- 



Adambulacral Ossicle. 



Ambulacral Ossicle, 



Radial Nerve. 

 Radial Blood-vessel 



Ampulla. 



The general cavity is the hypogastric coelom. 



On the under or oral side of the starfish the mouth is 

 situated in the centre. Along the centre of each arm there 

 runs a deep groove, the ambulacral groove. This is filled 

 wdth tw^o double rows of ambulacra or tube-feet, a number of 

 small processes terminating in suckers. They form the 

 locomotive organs of the animal. If the double rows be 

 pressed aside from the middle line, there can be seen a 

 delicate radial nerve in the middle of the groove. Further, 

 if the tube-feet be cut away it will be noticed that they 



