152 MANUAL OF ZOOLOGY. 



" 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 surface 

 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 " eyelids." 



The generative organs are in the form of ramified tubes, 

 arranged 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 dehiscence, the external medium in this latter case being 

 ultimately reached through the respiratory tubes. In their 

 development, the Asferoidea show the same general phenomena 

 as are characteristic of the class ; but the larvae are not pro- 

 vided with any continuous endoskeleton. In some Asteroids 

 the larval forms have been described under the name of Bipin- 

 naricR, and in these, as in the Pluteus of the Echinoids, a large 

 portion of the larva is cast off as useless. In Bifinnaria asteri- 

 gera (Sars) the digestive cavity is a simple sac which sends 

 no prolongations into the rays, and the mouth is inter-radial, 

 instead of being placed in the centre of the ambulacral system. 

 The mouth of the adult is at this stage closed by the soft exter- 

 nal skin of the larva. 



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

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

 bens) the disc is small, and is furnished with long, finger-like 

 rays, usually five in number. In the CribellcR (fig. 42) the 

 general shape of the body is very much the same. 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 



