A8TEROIDEA. 141 



CLASS ASTEROIDEA. 



The "Star-fishes" well represent the sub-kinijdom. The 

 common form is that of a star with five rays, wliich are prolonga- 

 tions of the body, — the viscera extending into them. Beneath 

 each ray is a groove from which is protruded the tnbe-feet. The 

 dorsal and lateral surfaces of the animal are covered by a cori- 

 aceous skin, strengthened by a net work of calcareous plates. 

 These ossicles have a persistent arrangement in the various 

 genera, so that they afford good evidence of the I'ank of the 

 owner among the Echinoderms, as do the bones of reptiles or 

 mammals among vertebrates. There is one ossicle, situated on 

 the side of the center of the disc, which differs from all the 

 others in being marked with radiating slits, and is called the 

 madreporiforin plate. It is the opening of the water-tube sys- 

 tem. In the center of the ventral surface is the toothless month, 

 surrounded by a bony ring. The eyes are generally situated at 

 the end of the rays. 



Fossil Star-fishes, though less common, have a wider range than 

 fossil Sea-Urchins. They are found in every geological period 

 from the Lower Silurian, attaining their maximum in the present 

 seas. The Palaeozoic species differ from recent ones in having 

 the plates perforated by pores. No family of Star-fishes has 

 become extinct. 



No. 434. [863, Cast]. Solaster Moretonensis, Forbes. 



This beautiful and well preserved fossil seems as if it were the head of a 

 Crinoid with out-spread arms, crushed flat. A nearer inspection shows that it 

 is really a Star-fish, resembling the living Uraster Jielianthus horn, the Pacific 

 coast of South America. From the Great Oolite, Windru.sh Quarry, Glouces- 

 tershire, England. Size, 6 x 6. 



No. 435. (1320, Cast]. Renaster margaritatus. 

 Lower Devonian, Lahnstein, Germany. 



