152 MANUAL OF ZOOLOGY. 



polygonal calcareous plates, accurately fitted together, and en- 

 closing all the viscera of the animal. The body was in most 

 cases permanently attached to the sea-bottom by means of 

 a jointed calcareous "column," or pedicle, but this was much 

 shorter than in the majority of Crinoids. Upon the upper 

 surface of the body were two, sometimes 

 three apertures, the functions of which have 

 been a matter of considerable controversy. 

 One of these is lateral in position, is de- 

 fended by a series of small valvular plates, 

 and is believed by some to be the mouth, 

 whilst by others it is asserted to have been 

 an ovarian aperture. The second opening 

 is central in position, and is believed by 

 Mr Billings to be the " ambulacral orifice," 

 as it is always in the centre of the arms 

 when these are present. The third aper- 

 ture is only occasionally present, and 

 Fig. 44. Cystidea. ECU- doubtless discharged the functions of an 



nospheerites aurantium, . , , _ 

 aCystidean from the anu S- 



Baia Limestone (Lower In some Cystoideo, there were no arms, 

 properly speaking, but only small pin- 

 nulae. In a second section true arms were present, but these 

 were bent backwards, and were immovably soldered down to 

 the body. In one single species (Comarocystites punctatus, Bil- 

 lings) the development has gone further, the arms being free, 

 and provided with lateral pinnulae, as in the true Crinoids. 



Many Cystideans are likewise provided with a system of 

 pores, or fissures, penetrating the plates of the body, and usu- 

 ally arranged in definite groups. These groups are termed 

 " pectinated rhombs," but their exact function is doubtful. 

 By Mr Billings, however, they are believed, and apparently 

 with good reason, to have admitted water to the body-cavity, 

 and to have thereby subserved a respiratory function. 



ORDER BLASTOIDEA. The members of this order, like those 

 of the preceding, are all extinct, and are entirely confined to 

 the Palaeozoic period. The body was fixed to the bottom of 

 the sea by means of a short-jointed pedicle ; it was globular 

 or oval in shape, and composed of solid polygonal calcareous 

 plates, firmly united together, and arranged in five inter-ambu- 

 lacral and as many ambulacral areas. (These ambulacral areas 

 are termed by M'Coy " pseud-ambulacra," upon the belief that 

 they were not pierced for tube-feet, but that they carried a 

 double row of little jointed tentacles or arms.) The ambula- 

 cral areas are petaloid in shape, having a deep furrow down the 



