Me 
56 E. Billings on the structure of the Crinoidea, ete. 
with one side of the animal, and is itself invested with a pretty 
strong skin, which is cover red with vibratile cilia. Its form 18, 
that of a plate rolled in at the margins till they meet. It feels 
gristy as if full of san eh we examine it with the micro- 
scope we find it to consist of minute calcareous plates, which 
are united into plates or joints, so that when the invest-— 
ing membrane is removed it has the appearance of a jointed 
column. Professor Ehrenberg remarked the former structure, 
Dr. Sharpey the latter: they are both ght. Both structures 
may be seen in the column of the common cross-fish.” (Forbes, 
British. eect . 73.) 
Pro h. Miller’ s work, “Uber den bau der Echinoder- 
men, Fee oe of the madreporic appendages of the different 
groups of the mvent Kchinodermata are described. In gen- 
one Se. extreit it resembles that of the Holothuri 
The convoluted plate of the Palzozoic Geneda: id the mad- 
reporic sacks. and tubes (or sand canals) of the recent Echino- 
derms, therefore, all agree in = gestae respects :— 
ave the same gener. ture. 
2. They are all appendages of ae Sead system. 
38. They are all attached to the same part of the system, that 
is to say, to the central point from whic the canals radiate. 
The above seems to me suflicient to make out at least a good 
prima facie case for the position I have assumed. When among 
the petrified remains of an extinct animal, we find an organ 
we has the same general form and structure, as has one that 
urs ms an existing species of the same zoological group, we 
ae th much probability of pane correct in our opinion, 
conclude that the two are homol ologous, even although we may 
not be able positively to see how that of the fossil is connected 
with any other part. But when, as in this instance, we can ac- 
tually see that it is an sg a ndage of sort organ, or system of 
organs rather, which is known to be th ogue of th 
5 
part with which that of the ores Pos oo is alvays correlated, 
hang. sviianee of 5 a very high epics on which to ground a 
cLUS1O: Mf ee 
