52 INVERTEBRATE ANIMALS. 



Group 143.— Genera ASTEROPSIS (Muller), and 

 ARCH ASTER, (Muller). Star-fishes, writes A. 

 Agassiz, have a singular mode of eating : they place 

 themselves over whatever they mean to feed upon, —as for 

 instance a cockle, — the back gradually rising as they arch 

 themselves above it ; they then turn the digestive sac 

 or stomach inside out, so as to enclose their prey com- 

 pletely, and proceed leisurely to suck out the animal 

 from the shell. Estimated number of species : recent, 

 11 ; fossil, 2. Green-sand. 



Ambulacral furrows, with two rows of tube-feet : anus 

 absent. 



Group 144.— Genera ASTROPECTEN (Linck) : LUIDIA 

 (Forbes). Three representatives of the present group are 

 found in British seas ; Asterias aurantiaca (Linnteus), 

 Astropecten (Linck), called by fishermen the Butthorn, 

 and two species of Luidia, one of which when full grown 

 measures two feet across, but specimens of this size are 

 with great difiiculty obtained entire,'Jowing to the habit 

 which Luidia possesses of voluntarily breaking itself 

 into little pieces when an attempt is^'made to remove it 

 from its natural element. This species, in common 

 with many others, has at the end of each ray a pigment 

 spot, which is probably a rudimentary organ of vision, 

 and is guarded by a movable eye-lid. Estimated 

 number of species : recent, 46 ; fossil, 20. Oolite and 

 upwards. 



