E. Billings on the structure of the Crinoidea, ete. dd 
formed of broad sacks, with a number of folds on one side, con- 
sist of ten simple cylindrical tubes connected together in five 
pairs. The only ditference between the structure of fig. 3 and 
fig. 4 is in the width of the tubes and in the absence of folds 
in the latter. These two forms are moreover connected by inter- 
mediate grades. Species with 11, 10, 8, 6, 5, 4 and 2 folds be- 
ing known, there is thus established a gradual transition from 
the broad petaloid form to the single cylindrical tube. 
Between the Cystidea and the Blastoidea the most important 
changes are, that in the latter the hydrospires become connected 
in pairs, and, also, are brought into direct communication with 
the pinnule. In the Paleozoic Crinoidea (or at least in many of 
them) concentration is carried one step further forward, the five 
pairs of hydrospires being here all connected together at the 
centre as in fic. 5. There is as yet no cesophageal ring, (as I 
understand it) but in its place the convoluted plate described in 
the excellent papers of Messrs. Meek and Worthen This organ, 
according to the authors, consists of a convoluted plate, resem- 
bling in form the shell of a Bulla or Scaphander. It is situated 
within the body of the Crinoid with its longer axis vertical and 
the upper end just under the centre of the ventral disc. Its 
lower extremity approaches but does not quite touch the bottom 
of the visceral cavity. Its walls are composed of minute poly- 
gonal plates or of an extremely delicate network of anastomos- 
ing fibres. The five ambulacral canals are attached to the upper 
extremity, radiate outward to the walls of the cup and are 
seen to pass through the ambulacral orifices outward into the 
grooves of the arms. (Ante, vol. xlviii, p. 
The ambulacral canals of the Crinoidea are, for the greater 
the rays: this is the madreporijorm tubercle or nucleus. on 
i i i is seen a curved calcareous col- 
. 
orifice into the circular vessel. Tt is connected by a membrane 
ae 
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