I 68 MANUAL OF ZOOLOGY. 



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 Ophiuroidta, but they are always an imme- 

 diate continuation 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. Aster iadce. Dorsal wart simple. 

 Section b. Ambulacra with t^vo rorvs of feet. 



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

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

 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 

 considered as belonging to the Asteroidea, to which they are 

 nearly allied. The body in the Ophiuroidea (fig. 61) is dis- 

 coidal, and is covered with granules, spines, or scales, but 

 pedicellarise are wanting. From the body which contains 

 all the viscera proceed long slender arms, which may be 

 simple or branched, but which do not contain any prolongations 

 from the stomach, nor have their under surface excavated into 

 ambulacral grooves. The arms, in fact, are not simple pro- 

 longations of the body, as in the Asteroidea, but are special 

 appendages, superadded for locomotive and prehensile pur- 

 poses. Each arm is enclosed by four rows of calcareous plates, 

 one on the dorsal surface, one on the ventral surface, and two 

 lateral. The lateral plates generally carry more or less well- 

 developed spines. In the centre of each arm is a chain of 

 quadrate ossicles, forming a central axis, and between this 

 axis and the row of ventral plates is placed the ambulacral 

 vessel. Each ossicle of the central chain is composed of two 

 symmetrical halves, but these are immovably articulated 

 together, and are not movable upon one another, as in the 

 Asteroidea. The mouth is situated in the centre of the inferior 

 surface of the body, is provided with a masticatory apparatus, 

 and is surrounded by tentacles. It opens directly into a sac- 

 like ciliated stomach, which is not continued into an intestine, 

 the mouth serving as an anal aperture. The stomach is 

 destitute of lateral diverticula. The reproductive organs are 



