The family of the Ophiuridæ, or Snake Stars, are widely distributed in the ocean. The genus Euryales with branching rays, and that of Ophiura with simple rays, comprising the Brittle and Sand Stars, are abundant in the British seas. In the sand stars there are cavities full of sand at the points from whence the rays diverge, which appear like warts on the surface of the disk. Their rays are exceedingly long, thin, and flexible; they have no central groove nor feet, but they are employed 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 themselves, firmly aided by spines or bristles on their edges. The Ophionyx has the addition of movable hooks beneath bristled spines. The rays are bent by the contraction of internal muscles, and extended again by the elasticity of the external leathery coat. The Ophiuridæ, 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 Ophiuridæ 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 separated 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 Stelleridæ; and respiratory organs, in the form of from two to four plates, or lamellæ, 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 aërate the blood. The colour of the star-fishes, as well as of other marine invertebrate animals, seems to be independent of light. The Ophiuridæ 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 coast of Norway were of brilliant hues—red, vermilion, white, and yellow. In general, both plants and animals of the lower kinds become of a sickly white when kept in darkness.