the Crinoidea, Cystidea, and Blastoidea. 413 



with a number of folds on one side, consist of ten siin])lc 

 cylindrical tubes connected together in five pairs. The only 

 difference between the structure of fig. 3 and fig. 4 is in tiie 

 "width of the tubes and in the absence of folds in the latter. 

 These two forms are, moreover, connected by intermediate 

 grades. Species with eleven, ten, eight, six, five, four, and 

 two folds being knoAvn, there is thus established a gradual 

 transition from the broad pctaloid form to the single cylindrical 

 tube. 



Between the Cystidea and the Blastoidea the most important 

 changes are that in the latter tlie hydrospires become connected 

 in pairs, and also are brought into direct communication with 

 the pinnulae. 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 centi-e, as in fig. 5. There is as yet no oesophageal ring (as 

 I understand it), but in its place the convoluted plate descriljed 

 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 

 disk. Its lower extremity approaches, but does not quite 

 touch, the bottom of the visceral cavity. Its walls are com- 

 posed of minute polygonal plates, or of an extremely delicate 

 network of anastomosing fibres. The five ambulaeral canals 

 are attached to the upper extremity, radiate outward to tlie 

 walls of the cup, and are seen to pass through the ambulaeral 

 orifices outward into the grooves of the arms. (Silliman's 

 Journ. vol. xlviii. p. 31.) 



The ambulaeral 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, truly 

 the homologues of those of the Echinodermata in general. In 

 the higher orders of this class the canals are usually more 

 sjjecialized than they are in the lower, being provided with 

 prehensive or locomotive organs. In all of the existing 

 orders, including the recent Crinoidea, we find an oesophageal 

 ring. 



To this organ, which is only a continuation of the canals, 

 are attached the madreporic appendages. These consist of 

 small sacs or slender tubes, varying greatly in form and num- 

 ber in the different genera. That of the starfish Aster acanthion 

 rxd)ens is thus described by Prof. E. Forbes : — ''On the dorsal 

 surface is seen a wai-t-like striated body placed laterally be- 

 tween two of the rays : this is the madreporifonn tubercle or 



