SAND-STABS AND STAR-FISHES. 



193 



beach. It is variable in color, but beautifully spotted with 

 pale and brown, its general hue being a brick-red. Am- 

 pliiura squamata Sars has long slender arms and is? 

 white ; it lives below tide-marks. The basket-fish, me- 

 dusa's head, or Astrophyton 

 Agassizii Stm., is of large 

 size, the disk being two in- 

 ches across, and the arms 

 subdividing into a great 

 number of tendril-like 

 branches. It lives from ten 

 to one hundred fathoms in 

 the Gulf of Maine. 



Ophiurans are widely dis- 

 tributed, and live at depths 

 between low- water mark and 

 two thousand fathoms. Fos- 

 sil Ophiurans do not occur 

 in formations older than the Upper Silurian, where they arc- 

 represented by the genera Protaster, Palceodiscus, Acroura, 

 and Eucladia ; genuine forms closely like those now living 

 appear in the muschelkalk beds of Europe (Middle Trias). 



Order 2. Asteridea. In the true star-fishes the arms are 

 direct prolongations of the disk, and the stomach and 



Fig. lS5.-OphiopkolMbellis, common Sand 

 star. After Morse. 



Fig. 136. Three forms of Star-fish, A , B, C, seen from above, showing the different 

 development of the ambulacra! and interambulacral areas. The ambulacra are indi- 

 cated by row8 of dots ; o, mouth; r, arm* ; ir, intermdial or iiiterambulacral areas. 

 V Pterusttr; B, Ooniodiscus; A, Asteriscuts. After Gegenbaur. 



ovaries or spermaries project into them, and there is a deep 

 ambulacral furrow, while the interambulacral spaces vary 

 much in development (Fig. 136); the feet are provided with 



