INVERTEBEATE ANIMALS. 51 



Ambulacral furrows with two rows of tube-feet ; auus dorsal. 



Group 140.— Genera CRIBELLA (Agassiz), SOLASTER 

 (Forbes), and allies. The genus Crihella forms a link 

 between the cross-fish, Uraster, and the sun-stars, 

 Solaster. Most of the Echiuoderms possess, and Crihella 

 exhibits very distinctly on its upper surface, near the 

 junction of two arms, a small aperture like the rose of 

 a watering-pot (madreporiform tubercle), which acts as 

 a filter for the supply of the water-vascular system. 

 Estimated number of species : recent, 53 ; British, 4 ; 

 fossil, 1. 



Group 141.— Genus PALMIPES (Linck), and allies. 

 In this group the arms or rays are slender, so that the 

 web which joins them constitutes the chief part of the 

 body, Palmipes, the Bird's-foot Sea-star, the thinnest 

 and flattest of the order, ranges from the Artie Seas to 

 the Mediterranean. Estimated number of species : 

 recent, 47 ; British, 1 ; fossil, 7. Higher Crag. 



^ Examples of Cidcita, Asteriscus, Oreaster, &c. 



Group 142.— Genera ASTROGONIUM (Mliller), and 

 GONIODISCUS (Miiller). The long, sharp, and 

 strong spines conspicuous in some of the species of 

 this group serve to protect the animal from the attacks 

 of enemies, or from the effects of sudden concussions. 

 The animal can lower its spines in any required direc- 

 tion, as river steamers lower their funnels when passing 

 under a bridge. Estimated number of species : recent, 

 30 ; British, 2 ; fossil, 34. Chalk and Higher Crag. 



11 Examples of Pctulia horrida, Antennea, &c. 



