ANNULOIDA: ECHINODERMATA. I5S 



terophyton, the Medusa-head star, the arras 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 

 folloAvs : — 



Family i. Ophiuridea. 



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

 Family 2. Asterophydia:. 



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



CHAPTER XXI. 



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 pri- 

 marily 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 svcdafie. is inferior, 

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

 however, which are given oft" from the radiating ambulacra! 

 canals of the Crinoidea, unlike those of the Echinoidea and 

 Asteroidea, are not used yi 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 ambu- 

 lacral or respiratory tubes. The arms are furnished with nu- 

 merous lateral branches or " pinnulse." The embryo is " free 

 and ciliated, and develops within itself a second larval form, 

 whichbecomes fixed by a peduncle." — (Huxley.) 



Of those Crinoidea which are permanently fixed to the sea- 

 . bottom by a jointed pedicle, there exist but a few Kving 

 forms, of which the best known is the Pentacrinus Caput- 

 Medusce. In this type of the Crinoidea — largely represented 



