annuloida: echinodermata. IS3 



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. Asteriadie. Dorsal wart simple. 

 Section b. Amublacra with two rows of feet. 



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

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



Family 3. PentaceroticUe. Body supported by roundish or elongated 

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

 pores between the tubercles. 



Family^ 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. 43) 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 am- 

 bulacral grooves. The arms, in fact, are not simple prolonga- 

 tions of the body, as in the Asteroidea, but are special appen- 

 dages, superadded for locomotive and prehensile purposes. 

 Each arm is enclosed by four rows of calcareous plates, one on 

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

 In the centre of each arm is a chain of quadrate ossicles, form- 

 ing 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 aper- 

 ture. The stomach is destitute of lateral diverticula. The 

 reproductive organs are situated near the bases of the arms, 



