254 ASTEKOIDEA AND OPHIUROIDEA. 



of the test. An ovarian aperture appears to be present in 

 one of the interradial spaces. This genus is only known as 

 occurring in the Lower Silurian, and it is clearly very closely 

 allied to Agelacrinus, though it has not yet been demonstrated 

 to have been attached to foreign bodies; and its other charac- 

 ters perhaps entitle it to generic distinction. Whether or 

 not we may place in the vicinity of Agelacrinus the extra- 

 ordinary Silurian genus Cydocystoides, cannot at present be 

 stated with any certainty. Salter would also refer here the 

 genera Echinocystites and Palceodiscus. 



ORDER III. OPHIUROIDEA. 



The Opliiuroidea are often grouped with the Asteroidea, 

 and the living members of the order are known commonly 

 as Brittle-stars and Sand-stars. They are distinguished from 

 the true Star-fishes by the fact that the " disc " contains all 

 the internal organs of the animal ; the " arms " are not grooved 

 inferiorly for the emission of ambulacral tube -feet ; and the 

 mouth is provided with a masticatory apparatus. The Ophiu- 

 roids are very conspicuously star-shaped, and consist of a 

 central "disc" and a series of radiating "arms" (fig. 146). 

 The " disc " is truly disc-shaped, and is covered with granules, 

 spines, or scales. From the disc proceed the arms, in the 

 form of long and slender processes, which may be simple or 

 branched, but which differ from the arms of Star-fishes in 

 not containing any prolongations from the stomach, and in 

 never having their under surfaces furrowed by ambulacral 

 grooves. The arms, in fact, are special processes superadded 

 for the purposes of prehension and locomotion, and rendered 

 necessary by the fact that the ambulacral system takes no 

 part in the function of locomotion, as it does in the Star- 

 fishes. A madreporiform tubercle, however, is present, and 

 is placed on the inferior surface of the body, being com- 

 monly concealed by one of the plates surrounding the 

 mouth. The mouth, as in the Star-fishes, is placed in the 

 centre of the lower surface of the disc ; but the stomach 

 terminates blindly ; and there is, therefore, no anal aperture. 



Each arm is furnished with an internal and an external 



