I 70 MANUAL OF ZOOLOGY. 



with five obtuse angles, and the arms are prehensile. In 

 Asterophyton, the Medusa-head star, the arms are divided from 

 the base, first dichotomously, and then into many branches. 

 In Ophiura, the sand-star, the arms serve for reptation (creep- 

 ing), and are undivided, often exceeding the diameter of the 

 disc many times in length. 



The order Ophiuroidea may be divided into two families, as 

 follows : 



Family I. Ophiuridea. 



Genital fissures two or four in number. Arms five, always simple. 

 Family 2. Asierophydia. 



Genital fissures ten in number. Arms five, simple or branched. 



CHAPTER XXL 



CRINOIDEA, CYSTOIDEA, AND BLASTOIDEA. 



ORDER CRINOIDEA. The members of this order are Echino- 

 dermata, in which the body is fixed, during the whole or a portion 

 of the existence of the animal, to the sea-bottom by means of a 

 longer or shorter, jointed, and flexible stalk. The body is dis- 

 tinct, composed of articulated calcareous plates, bursiform, 

 or cup-shaped, and provided with solid arms, which are 

 primarily from five to ten in number, are independent of the 

 visceral cavity, and are grooved on their upper surfaces for the 

 ambulacra. (The position of the body being reversed, the 

 upper surface is ventral ; whilst the dorsal surface is inferior, 

 and gives origin to the pedicle.) The tubular processes, 

 however, which are given off from the radiating ambulacral 

 canals of the Crinoidea, unlike those of the Echinoidea and 

 Asteroidea, are not used in locomotion, but have probably a 

 respiratory function. The mouth is central, and looks upwards, 

 an anal aperture being sometimes present, sometimes absent. 

 The ovaries are situated beneath the skin in the grooves on 

 the ventral surfaces of the arms or pinnules, as are also the 

 ambulacral or respiratory tubes. The arms are furnished with 

 numerous lateral branches or " pinnulae." The embryo is "free 



