130 MANUAL OF ZOOLOGY. 



ray. A distinct intestine and anus may, or may not, be pre- 

 sent ; but the anus is sometimes wanting. 



The ambnlacral system is essentially the same as in the 

 Echinoidea, and is connected with the exterior by means of 

 the ' madreporiform tubercle,' or 'nucleus,' two, three, or more 

 of these being occasionally present. The ambulacral tube-feet 

 are arranged in two or four rows, along grooves in the under 

 surface of the arms. 



The blood-vascular system consists, as in the Echinus, of 

 two circular vessels, one round the intestine, and one round 

 the gullet, with a dilated tube, or heart, intervening between 

 them. There are no distinct respiratory organs, but the sur- 

 faces of the viscera are abundantly supplied with cilia, and 

 doubtless subserve respiration ; the sea- water being freely ad- 

 mitted into the general body-cavity by means of numerous 

 contractile ciliated tubes, which project from the dorsal sur- 

 face of the body, and are perforated at their free extremities. 

 (Owen.) 



The nervous system consists of a gangliated cord, surround- 

 ing the mouth, and sending filaments to each of the rays. At 

 the extremity of each ray is a pigment- spot, corresponding to 

 one of the ocelli of an Echinus, and, like it, supposed to be a 

 rudimentary organ of vision. The eyes are often surrounded 

 by circles of movable spines, called ' eye-lids.' 



The generative organs are in the form of ramified tubes, ar- 

 ranged in pairs in each ray, and emitting their products either 

 into the surrounding medium, by means of efferent ducts which 

 open round the mouth, or into the general body-cavity, by cle- 

 hiscence, the external medium in this latter case being ulti- 

 mately reached through the respiratory tubes. In their deve- 

 lopment, the Asteroidea show the same general phenomena as 

 are characteristic of the class ; but the larvae are not provided 

 with any continuous endoskeleton. 



The general shape of the body varies a good deal in different 

 members of the order. In the common star-fish (Urastcr ru- 

 bens, Jig. 34) the disc is small, and is furnished with long, fin- 

 ger-like rays, usually five in number. In the Solasters the 

 disc is large and well marked, and the rays are from twelve to 

 fifteen in number, and are narrow and short (about half the 

 length of the diameter of the body). In the Goniasters the 

 body is in the form of a pentagonal disc, flattened on both 

 sides; the true ' disc ' and rays being only visible on the under 

 surface of the body. In none of the true star-fishes, however, 

 are the arms ever sharply separated from the disc, as in the 

 Ophiuroidea, but they are always an immediate continuation 

 of it. 



