140 LOWER INVERTEBRATES. 



spaces, a cone of small plates called the pyramid, with a central aperture. In ordi- 

 nary cystids, pinnules are present, which, when the arms are absent, are sessile on the 

 radial plates. An aj)erture placed in the centre of the calyx at the point of conver- 

 gence of the ambulacra, is usually regarded as the mouth ; a second small one on one 

 side of this is supposed to be the anus ; while the opening in the centre of the pyramid 

 is considered to be the reproductive aperture. Thus the Cystidea differ from other 

 echinoderms, the holothurians excepted, in the presence of only one genital aperture. 



Ilyponome sarsii, from Torres Strait, looks like a small star-fish or Euryale. It 

 has a disc, convex on the oral surface, and flattened on the other, which shows no 

 trace of a calyx, but is covered with. a soft and smooth brownish skin. The rays are 

 five, broad and short, yet each divided into two branches, ending in four very short, 

 rounded lobes. The oral surface is covered with rather small, thick-set, irregular 

 scales, M'ith rosettes of six or se^•en larger ones here and there. These scales extend 

 on to the dorsal sui-face between the rays, forming triangular spaces pointing to the 

 centre of the disc, and thus reducing the spaces covered by skin to a regular star. 

 The rays have narrow channels, protected by marginal scales, but have no pinnules. 

 Upon the disc the marginal scales form a vault over the channels, and the mouth itself 

 is hidden by the skin. There is a conical anal funnel in one of the intei'-radial spaces 

 on the oral surface, and an area in the middle of the dorsal surface is studded by 

 minute pores. Funnel-like passages leading to a concealed mouth are found in pala3- 

 ozoic Brachiata and cystideans. The absence of a calyx excludes Ilyponome from 

 the former; and it in some respects recalls the genus Agelacrinites among fossil 

 cystids. 



In opposition to the usual idea about these fossils, Mr. Billings maintains that the 

 large lateral aperture of a eystidean is the mouth, and the small apical orifice aii 

 ambulacral aperture. When there is no separate anal aperture, the large lateral aper- 

 ture is both mouth and anus. 



The Cystidea first occur in the cambrian, attained their greatest development in the 

 Silurian, and were mostly extinct in the carboniferous period. 



Oeder III. — BRACHIATA. 



This order was represented by a multiplicit}- of forms in the past geological ages, 

 and has latterly been shown to be far more abundant in species in the present age than 

 was supposed. As the name implies, all the species are provided with arms. These 

 arms are composed of numerous calcareous joints, and contain only a small proportion 

 of living matter. Existing crinoids belong to two distinct series. The first of these 

 contains forms which are permanently attached by a stalk to the sea-bottom, while the 

 second is thus attached while young, but finally becomes detached, the cup or calyx 

 with its arms and a pear-shaped centro-dorsal tubercle, formed of the coalesced upper 

 stem-joints, floating off freely. In this stage the animal greatly resembles a star-fish 

 in appearance, but its anatomy is very different. The stalked crinoids, which are prin- 

 cipally natives of deep water, may be considered as the representatives of the extinct 

 stone-lilies, while those which become fi-ee belong to a more recent and more highly 

 develojDed type. 



The characters which distinguish the crinoids from other echinoderms may be 

 briefly summed up as follows : The animal is attached, during the early portion or the 

 whole of its life, by a stem composed of more or less numerous joints. To the top of. 



