ANNULOIDA : ECHINODEEMATA. 



135 



mal creeps about by means of its pinnated arms. The mouth 

 is central in position, and the anus, which in some species 

 forms a tubular projection, is situated on one side. Both the 

 arms and the lateral pinnulee are grooved on their ventral sur- 

 faces for the ambulacra ; and the pinnules also serve for the 

 support of the reproductive organs. It is extremely doubtful 

 if the ambulacra! system, in the adult, has any communication 

 with the exterior. 



ORDER CYSTOIDEA (Cystidea). The members of this order 

 are all extinct,* and are entirely confined to the Paleozoic 

 period. The body (fig. 37) was more or less spherical, and was 

 protected by an external skeleton, composed of numerous 

 polygonal 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. 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, 

 w r hilst 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 

 doubtless discharged the functions of an 

 anus. 



In some Cystoidea 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, Billings) the development has gone 

 further the arms being free, and provided with lateral 

 pinnulfie, as in the true Crinoids. 



Many Cystideans are likewise provided with a system of 

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



* Recently Professor Lo'ven has described a singular Australian 

 Echinoderm as being most closely allied to, if not truly referable to, the 

 order Cystoidea. He has named this curious form Ht/ponome Sarsi, and 

 believes it to be nearly related to the Cystidean genus Agelacrinites. 



ip. 37. Cystidea. 

 Echinosphcerites au- 

 rantium, a Cystidean 

 from the Bala Lime- 

 stone (Lower Sihi- 

 rian). 



