CH. XIV] 



ASTERIAS 



327 



The star-fish creeps about with its mouth downwards : its 

 motion is effected by means of numerous delicate 



okeleton. 



semi-transparent tentacles. These are situated in 

 five grooves which run along the under side of the arms and con- 

 verge towards the mouth, where they merge into a depression 



Fm. 150. 



Oral view of Echinaster sentus with tube-feet-extended x about 1. 

 From Agassiz. 



surrounding that opening. These grooves are termed the ambu- 

 lacral grooves: the tentacles situated in them are called the 

 tube-feet, and the depressed space round the mouth in which 

 all the grooves unite is called the buccal membrane (Lat. bucca, 

 the cheek) or the peristoine (Gr. Tre/ot, around, and o-rd/xa, mouth). 



