ANNULOIDA : ECHINODERMATA. 



159 



Fig. 46. — Cystidea. Ecki- 

 jiosfi/uEritesanrantivTn, 

 a Cystidean from the 

 Bala Limestone (Lower 

 Silurian). 



In a second sec- 



npper surface of the body were two, sometimes three aper- 

 tures, the functions of which have been a matter of considerable 

 controversy. One of these is lateral in position, is defended 

 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 view advocated by 

 Mr Billings, that this aperture was the 

 mouth, appears from every point of view 

 to be most probably correct ; or rather it 

 was oro-anal, as was also the proboscis of 

 the Palaeocrinoids. 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 

 doubtless discharged the functions of an 

 anus. 



In some Cystoidea there were no arms, 

 properly speaking, but only small pinnulse. 

 tion true arms were present, but these were bent backwards, 

 and were immovably soldered down to the body. In one 

 single species {Comarocystites punctatus, Billings) the develop- 

 ment 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 coniposed 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 ambuJa- 

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

 centre, and striated transversely. They converge to the mouth, 



