ANNULOIDA : ECHINODEEMATA. 129 



CHAPTER XX. 



ASTEROIDEA AND OPHIUROIDEA. 



ORDER ASTEROIDEA (Stellerida). This order comprises the 

 ordinary star-fishes, and is defined by the following charac- 

 ters : The body is star-shaped, and consists of a central body 

 or ' disc,' surrounded by five or more lobes, or * arms,' which 

 radiate from the body, are hollow, and contain prolongations 

 of the viscera. The body is not enclosed in an immovable box, 

 as in the Echinoidea, but the integument ('perisome') is cori- 

 aceous, and is strengthened by irregular calcareous plates, or 

 studded by calcareous spines. No dental apparatus is present. 

 The mouth is inferior, and central in position ; the anus either 

 absent or dorsal. The ambulacral tube-feet are protruded 

 from grooves on the under surface of the rays. The larva is 

 vermiform and has no skeleton. 



The skeleton of the Asteroidea is composed of a vast number 

 of small calcareous plates, or ossicula, united together by the 

 coriaceous perisome, so as to form a species of chain- armour. 

 Besides these, the integument is abundantly supplied with 

 spices, tubercles, and 'pedicellarias.' The upper surface of a 

 star-fish corresponds to the combined inter- ambulacral areas 

 of an Echinus, and exhibits the aperture of the anus (when 

 present), and the ' madreporiform tubercle,' which is situated 

 near the angle between two rays. The inferior or ventral sur- 

 face corresponds to the ambulacral areas of an Echinus, and 

 exhibits the mouth and ambulacral grooves. 



Fig. 34. Asteroidea. Uraster mbens, the common Star-fish or Cross-fish. 



The mouth is central in position, and is not provided with 

 teeth ; it leads, by a short gullet, into a large stomach, from 

 which a pair of sacculated diverticula are prolonged into each 



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