CYSTOIDEA. 131 



by one of the lateral surfaces, instead of by the dorsal surface. 

 In one Cystidean only (viz., Comarocystites punctatus, Billings) 

 are there free arms as in the true Crinoids ; but further re- 

 searches will doubtless show that these appendages existed in 

 other species as well. In Comarocystites, however, the arms 

 differ from those of the Crinoids in being only four in number, 

 in not subdividing (though they carry lateral pinnae), and in 

 arising directly from the summit of the calyx. 



Upon the upper surface of the calyx in the Cystideans are 

 two, or sometimes three, apertures, the functions and nature 

 of which have given rise to considerable controversy. The 

 best known of these is a large opening which is pierced in one 

 side of the calyx, usually near the middle of the body, but 

 sometimes approximated to either the apex or the base. This 

 aperture is mostly defended by a "valvular pyramid;" or, in 

 other words, by a series of small plates, arranged in a pyra- 

 midal manner, and serving for the closure of the opening. 

 Much difference of opinion has prevailed as to the true nature 

 of this orifice. Von Buch believed that it was an " ovarian 

 aperture ; " Mr Billings regards it as discharging the functions 

 of both the mouth and anus ; whilst Professor Wyville Thom- 

 son, Mr Salter, and other high authorities, regard it as being 

 the anus, and as corresponding with the proboscis of the 

 Crinoids. That it is not an " ovarian orifice " may be re- 

 garded as certain, so that the question is narrowed to its 

 being the anus alone, or an " oro-anal" orifice. In the living 

 Le.tkia mirabilis, one of the Sea-urchins, both the mouth and 

 vent are closed by converging triangular valves, which doubt- 

 less correspond with the " pyramid " of the 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. 



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 shell to 

 the exterior. The analogies of recent forms, however, would 

 support the view that this aperture is the mouth, in which case 



