430 THE CANADIAN NATURALIST. [Dec. 



those of P. Schultzii, the structure exposed will be that represented 

 ia the diagram, fig. 3. In Pentremites as in Cadaster, the five 

 hydrospires are divided into ten equal parts by the five rays, a\, 

 a2, &c. In Cadaster these ten parts remain entirely separate 

 from each other, but in Pentremites they are re-united in pairs, 

 the two in each iuterradial space, being so connected, at their 

 inner angles, that their internal cavities open out to the exterior 

 through a single orifice or spiracle (s, figs. 3 and 4). This is 

 best shown in fig. 4, intended to represent the structure of P. 

 ellepticus (Sowerby) as described by Mr. Rofe, Geol. Mag., vol. 

 ii, p. 249. In this species the hydrospires instead of being 

 formed of broad sacks, with a number of folds on one side, consist 

 of ten simple cylindrical tubes connected together in five pairs. 

 The only difference 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 intermediate grades. 

 Species with 11, 10, 8, 6, 5, 4 and 2 folds being 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 

 pinnulee. In the Palaeozoic 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 fig. 5. There is as yet no oesophageal 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, resembling 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 polygonal plates, or of an extremely 

 delicate net work of anastomosing 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. 



The ambulacral canals of the Crinoidea are, for the greater 

 part, respiratory in their function. They are, however, as most 

 naturalists who have studied their structure will admit, trulv the 



