CHAPTER XIII 



THE ECHINODERMS 



305. Echinoderma. This group (a phylum) contains the 

 starfisftes, sea-urchins, crinoids or sea-lilies, and sea-cucum- 

 bers, all very peculiar animals widely different from those 

 of all other phyla. All members of the phylum are inhabit- 

 ants of the seas. No other phylum of animals is exclusively 

 marine ; and no one knows any good reason why echinoderms 

 have not migrated up rivers. This is another of thousands 

 of well-known biological facts which appear inexplicable. 

 All the forms named above have peculiar spines on the 

 skin (best developed in sea-urchins) ; and the name Echino- 

 derma means spiny skin. We shall have time in this course 

 for only a brief account of some common members of this 

 group. 



306. Starfish. (D) A starfish illustrates the general plan of 

 echinoderm structure. The most common species of our Atlantic 

 coast has a central disc and five flexible arms, and appears radially 

 symmetrical. In 



fact, naturalists in 

 the early part of 

 the nineteenth cen- 

 tury thought they 

 were related to 

 jelly-fishes and 



other coelenterates. A B C 



Later studies have -piG. 114. Outlines of three forms of starfishes, ra, 

 shown that star- mouth in center of each ; r, the groove containing 

 fishes and all other the foot-suckers. (After Gegenbaur.) 

 echinoderms are bi- 

 laterally symmetrical with reference to the internal organs. The 

 median plane is marked by a small pore-plate on the upper surface 

 near the angle between two arms, and a knife passed through this 



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