252 THE MICROSCOPE. 



occur in all the more recent marine strata of the earth's crust. They 

 are commonly called Sand Stars, or Brittle Stars, fig. 110. 



The family Asteridce, the common Star-fish, so abundant on our 

 coasts, is an example. In this family the arms appear to be merely 

 prolongations of the disk j they are usually five in number, and the 

 plates from which the ambulacra are exserted are placed in deep fur- 

 rows, which run along the lower surface of the arms. 



HOLOTHURI^E, OR SEA- CUCUMBERS. 



In this order the body , acquires a worm-like form. The radiate 

 structure is in fact scarcely recognisable in these animals, except in 

 the arrangement of the tentacles which surround the mouth. The 

 body is always more or less elongated, with the mouth at one end and 

 the anal opening at the other; the calcareous deposit in the skin is 

 reduced to scattered granules ; and in one family the ambulacra are 

 entirely wanting. 



This order is divided into two families. The first, the Synaptidce, 

 are characterised by the total absence of ambulacra, the motions of the 

 animals being assisted by peculiar anchor-like processes of the cal- 

 careous grains which project from the skin, and roughen the surface of 

 the animal. The spiculum represented at fig. 108 is serrated on the 

 convex edge, and the opposite extremity is recurved, and appears to 

 be connected in some peculiar way with the oval plate upon which it 

 lies. It is a beautiful object for polarised light. 



In the Holothurice, on the contrary, the ambulacra, although short, 

 exactly resemble those of the. other Echino-dermata in their structure 

 and action. The mouth is surrounded by a ring of calcareous plates 

 serving for the attachment of the longitudinal muscles, by which the 

 contractions of the body are effected. These animals inhabit the seas 

 of most parts of the world. Some of them are eaten even by Euro- 

 pean populations ; and the Trepang (Holothuria edulis) is an article of 

 luxury amongst the Chinese. A few living specimens have been lately 

 introduced into the tanks at the Zoological Gardens, where they are 

 established as objects of great interest to the naturalist. 



