107 



that in all the Cystidea, (and in none is it more obvious than in Caryo- 

 crinus), there was no connection between the hydrospires and the pinnu- 

 Ise. The main difference (so far as regards the evidence of the presence 

 or absence of such a connection) between Caryocrimis and Cadaster, con- 

 sists in this, that in the former the arms are erect and do not touch the 

 hydrospires, whereas in the latter they are recumbent and He back upon 

 them. Each of the arms of Codastcr has a fine ambulacra! groove, and 

 all of the grooves terminate in a single central aperture. But as this aper- 

 ture was covered over by a thin plated integument, as in the Blastoidea, 

 I have not shown it in the diagram, but only the five pores, p. 



No one who compares a Codastcr with a Pcntremites (the internal 

 structure of the latter being visible) can doubt that the hydrospires of the 

 two genera are perfectly homologous organs. If we grind olf the test of 

 a species of the latter genus, selecting one for the purpose which has broad 

 petaloid ambulacra such as those o p. ScJmltzii^ the structure exposed 

 will be that represented in the diagram, fig. 66. In Fentremites, as in 

 Codaster, the five hydrospires are divided into ten equal parts by the five 

 rays, al, a2, &c. In Codastcr these ten parts remain entirely separate 

 from each other, but in Pcntremites they are re-united in pairs, the two 

 in each interradial space being so connected, at their inner angles, that 

 their internal cavities open out to the exterior through a single orifice or 

 spiracle (5 figs. 66and 67). This is best shown in fig. 67 intended to repre- 

 sent the structure of P. ellepticus (Sowerby) as described by Mr. Rofe, 

 Geol. Mag., vol. ii, p. 249. In this species the hydrospires instead of be- 

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

 ten simple cylindrical tubes connected together in five pairs. The only 

 difference between the structure of fig. 66 and fig. 67 is in the width of the 

 tubes and in the absence of folds in the latter. These two forms are more 

 over 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 Cyst idea 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 pinnulce. In the 

 Palgeozoic Crinoidea (or at least in many of them) concentration is carried 

 one step further forward, the five pairs of hydrospires being here all con- 

 nected together at the centre as in fig. 68. 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 Bullz or Scaphander. It is situated within the body 



