INVERTEBEATE ANIMALS. 



51 



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 

 (LiNN^us), Astropecten (Linck), called by fishermen 

 the Butthorn, and two species of Luidia, one of which 

 when full grown measures two feet across, but speci- 

 mens of this size are with great difficulty obtained 

 entire, owing 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. 



The Mark iy indicates specimens or other illustrations exhibited in the Cases. 



Sub-class ECHINOIDA. Sea-Urchins. 

 Ixjvof, a hedgehog; IjSoj, form. 

 Family CIDARID^. ^llapii, a Persian turban. 



Group 145.— Genus CIDARIS (Lamarck) and allies. The 

 globose Cidaridce differ widely in appearance from the 

 long-armed Asterida of the preceding order ; but if the 

 arms of a star-fish were turned up over its back till the 

 tips nearly met, and if the sides of the turned-up arms 

 were united by plates bearing tubercles crowned with long 

 spines, the morphological affinity between the star-fish 



