146 MANUAL OF ZOOLOGY. 



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 Aster oidea 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 Bi- 

 pinnaria, and in these, as in the Pluteus of the Echinoids, a 

 large portion of the larva is cast off as useless. 



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 Cribellce (fig. 40) 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 

 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 con- 

 tinuation of it. 



The order Asteroidea has been divided by Dr Gray as fol- 

 lows : 



ORDER ASTEROIDEA. 



Section a. Ambulacra with four rows of feet. 



Family I. Asteriada. Dorsal wart simple. 

 Section b. Ambulacra with two rows of feet. 



Family 2. Astropectinidce. Back flattish, netted with numerous tuber- 

 cles, crowned with radiating spines at the tip, called "paxillae." 



Family 3. Pentacerotida. Body supported by roundish or elongated 

 pieces, covered with a smooth or granular skin, pierced with 

 minute pores between the tubercles. 



Family 4. Asterinidce. Body discoidal or pyramidal ; sharp-edged ; 

 skeleton formed of flattish, imbricate plates ; dorsal wart single, 

 rarely double. 



ORDER OPHIUROIDEA. This order comprises the small but 

 familiar group of the "Brittle-stars" and "Sand-stars," often 



