2O4 MANUAL OF ZOOLOGY. 



and dorsally with an anal plexus, from which branches are dis- 

 tributed to the genital glands. There are no distinct respira- 

 tory organs, but the surfaces of the viscera are abundantly 



Fig. 99. Section of the ray of Uraster rribens. a a Ambulacral ossicles ; b Position 

 of the ambulacral vessel ; c c Plates of the external skeleton ; n Nerve-cord. The 

 dotted lines show the tube-feet proceeding from the ambulacral vessel. 



supplied with cilia, and doubtless subserve respiration; the 

 sea-water being freely admitted into the general body-cavity 

 by means of numerous contractile ciliated tubes, which project 

 from the dorsal surface of the body. 



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 

 into the surrounding medium by means of efferent ducts which 

 open round the mouth. In their development, the Asteroidea 

 show the same general phenomena as are characteristic of the 

 class ; but the larvae are not provided with any continuous 

 endoskeleton. In some Asteroids the larval forms have side- 

 lappets, and have been described under the name of Bipin- 

 naria; and in these, as in the Pluteiis of the Echinoids, a large 

 portion of the larva is cast off as useless. In Bipinnaria 

 asterigem (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 external skin of the larva. In other Asteroids the larvae 

 have three anterior vermiform processes, and are known as 

 Brachiolaria. 



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



