176 MANUAL OF ZOOLOGY. 



gonal calcareous plates, accurately fitted together, and enclosing 

 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, and was rarely altogether absent. 

 Upon the upper surface of the body were two, sometimes three 

 apertures, the functions of which have been a matter of consider- 

 able 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 is that this aperture was the 

 mouth, or rather that it was oro-anal, as was 

 also the proboscis of the Palseocrinoids. 

 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 aperture is only occasionally 

 present, and doubtless discharged the func- 

 ]?i g . 65. Cystidea. Ecki- tions of an anus. High authorities, how- 

 nospfuerites anrantium, ever regard the lateral valvular aperture 



a Cystidean from the , . . 



Baia Limestone (Lower as the anus, and the central aperture as 

 the mouth a view which is supported 

 by the analogies of recent forms. 



In some Cystoidea there were no arms, properly speaking, but 

 only small pinnulae. 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, Billings) the development has gone further, the arms 

 being free, and provided with lateral pinnulse, as in the true 

 Crinoids. 



Many Cystideans are likewise provided with a system of 

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

 usually 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 



