CYSTOIDEA. 289 



Cystideans. This recent form, however, is not sufficient to 

 decide the present question, since in it both the mouth and 

 the anus exhibit this valvular apparatus. Upon the whole, 

 therefore, this question must be regarded as undecided, 

 though the analogies of recent forms would lead to the belief 

 that the " pyramid " of the Cystideans is truly the anus, and 

 that the mouth must be sought for between the bases of 

 the arms, when these are present. 1 



A second aperture is placed near the centre of the summit 

 of the calyx, between the bases of the arms, when these 

 exist. This opening has not been universally detected, and 

 its true nature is doubtful. By Mr Billings it is believed to 

 be what he terms an "ambulacral orifice" i.e., an aperture 

 by which the ambulacral vessels passed from the interior of 

 the calyx to the exterior. The analogies of recent forms, 

 however, would support the view that this aperture is the 

 mouth, in which case the " pyramid " must be the anus. 

 There is, however, in some cases a third aperture of small 

 size, always placed near the apex, and this has been regarded 

 as being truly the anus. 



Many Cystideans were further provided with a system of 

 minute pores or fissures, penetrating the plates of the calyx, 

 and often arranged in definite groups. These groups are 

 known as " pectinated rhombs " or " hydrospires," and their 

 exact function is not absolutely certain ; though there can 

 be little hesitation in accepting the views of Billings and 

 Eofe that they acted as respiratory organs. Their form 

 differs considerably in different genera. In many cases the 

 porous surfaces or " pectinated rhombs " have the figure of 

 diamond -shaped areas, divided into two halves, of which one 

 belongs to one plate of the perisome, and the other to the 

 contiguous plate (fig. 173). Generally, if not invariably, the 

 pores upon the one half of the rhomb are connected each 



1 Sir Wyville Thomson has recently shown that the mouth in the living 

 Hyocrinus and Rhizocrinus is protected by a pyramid of five calcareous valves ; 

 but this aperture is central, and thus differs from the laterally - placed 

 "pyramid" of the Cystideans. It seems not improbable that the mouth of 

 the Cystideans was not only central in position, but that it resembled the 

 mouth of the Palseocrinoids in being hidden from view by the calcareous 

 plates covering the vault of the calyx. 



VOL. I. T 



