416 
Proceedings of the Royal Society 
composed of particles of organic matter, the shields of diatoms, 
and the shells of minute foraminifera. The mode of nutrition 
may be readily observed in Antedon , which will live for months in 
a tank. The animal rests attached by its dorsal cirri, with its 
arms expanded like the petals of a full-blown flower. A current 
of sea- water, bearing organic particles, is carried by the cilia along 
the brachial grooves into the mouth, the water is exhausted in the 
alimentary canal of its assimilable matter, and is finally ejected 
at the anal orifice. The length and direction of the anal tube 
prevents the exhausted water and the foecal matter from returning 
at once into the ciliated passages. 
In the probably extinct family Cyathocrinidse, and notably in 
the genus Cyatkocrinus , which I take as the type of the Palaeozoic 
group, the so-called Crinoidea tessellata, the arrangement, up to 
a certain point, is much the same. There is a widely-expanded 
crown of branching arms, deeply grooved, which doubtless performed 
the same functions as the grooved arms of Pentacrinus ; but the 
grooves stop short at the edge of the disc, and there is no central 
opening, the only visible apertures being a tube, sometimes of 
extreme length, rising from the surface of the disc in one of 
the interradial spaces, which is usually greatly enlarged for its 
accommodation by the intercalation of additional perisomatic plates, 
and a small tunnel-like opening through the perisom of the edge 
of the disc opposite the base of each of the arms, in continuation 
of the groove of. the arm. The functions of these openings, and 
the mode of nutrition of the crinoid having this structure, has 
been the subject of much controversy. 
The author had lately had an opportunity of examining some 
very remarkable specimens of Cyatkocrinus arthriticus, procured by 
Mr Charles Ketley from the upper Silurians of Wenlock, and a 
number of wonderfully perfect examples of species of the genera 
Actinocrinus , Platycrinus , and others, for which he was indebted to 
the liberality of Mr Charles Wachsmuth of Burlington, Ohio, and 
Mr Sidney Lyon of Jeffersonville, Indiana; and he had also had 
the advantage of studying photographs of plates, showing the 
internal structure of fossil crinoids, about to be published by Messrs 
Meek and Worthen, State Geologists for Illinois. A careful 
examination of all these, taken in connection with the description 
