INVERTEBRATE ANIMALS. 49 



Order ASTEKOIDEA. ao-r^p, a star; slhc, 

 form, 

 Ambulacral furrows with four rows of tube-feet ; anus dorsal. 



Group 139.— Genera ASTERACANTHION (Muller) ; and 

 HELIASTEE (Gray). This group includes the com- 

 mon star-fish or cross-fish, found abundantly on the 

 shores of the Mersey. The arms of star-fishes have, on 

 the under side, furrows pierced with rows of openings 

 through which can be extended or retracted locomotive 

 organs called tube-feet. A complicated apparatus (the 

 water-vascular system) fills or empties, and thus pro- 

 trudes or withdraws, these tube-feet, or any portion of 

 them, at the will of the animal. Estimated number of 

 species ; recent, 50 : British, 4 ; fossil, 9. Silurian 

 and upwards. In this, as in most groups of the order, 

 the collection is indebted for fine examples to the liberal 

 contributions of Professor Agassiz. 



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



Group 140.— Genera CRIBELLA (Agassiz), SOLASTER 

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

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

 Solaster. Most of the Echinoderms possess, and Cribella 

 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. \ 



