SECT. vi. ECHINODERMATA. 173 



Their rays are exceedingly long, thin, and flexible; 

 they have no central groove nor feet, but they are em- 

 ployed as organs of locomotion and prehension, for by 

 their alternate strokes the sand stars can elevate or 

 depress themselves in the water, creep on the bottom, 

 and by twisting them round objects they can fix them- 

 selves, firmly aided by spines or bristles on their edges. 

 The Ophionyx has the addition of movable hooks be- 

 neath bristled spines. The rays are bent by the con- 

 traction of internal muscles, and extended again by the 

 elasticity of the external leathery coat. The Ophiuridse, 

 like the Luidia fragilissima belonging to the preceding 

 order, cast off a ray if touched, and even all the five if 

 rudely handled ; but they can replace them with as much 

 ease. If only a fragment of a disk remains attached to 

 a ray the whole animal may be reproduced. 



The Ophiuridse have an internal calcareous skeleton 

 or framework, in the form of spicules, scattered in their 

 tissues. They have a capacious mouth with tentacles 

 and ten small chisel-shaped teeth, five on each side, 

 which meet and close the mouth. The mouth is sepa- 

 rated from the stomach by a circular muscle that opens 

 and shuts the passage, but no canal diverges from the 

 stomach through the rays. The nervous system and the 

 circulation of the blood are similar to those in the Stel- 

 leridse ; and respiratory organs, in the form of from two 

 to four plates, or lamellae, project from each of the spaces 

 between the bases of the rays into the central cavity, by 

 which sea-water has free access to bathe the digestive 

 organs and aerate the blood. 



The colour of the star-fishes, as well as of other ma- 

 rine invertebrate animals, seems to be independent of 

 light. The Ophiuridse that had been living at a depth 

 of 1,260 fathoms in the North Atlantic were coloured, 

 though not a ray of light could reach their dark home, 

 and those dredged up from 100 to 300 fathoms on the 



