THE IIYDRONPIUKS \M) sl'III U'l.F.s. 79 



verify the suggestions of McCoy and Hall respecting the close similarity between the 

 hydrospires of Codaster and the pectinated rhombs of aCystid; and be suggested 

 " the possibility of their being respiratory sacs, lined with cilia, and constructed of a 



porous test, through which air from the water could pass by diffusion." Meek 

 and Worthen * writing in 1869 doubted the respiratory nature of the hydrospires 

 because of the absence of any direct communication between their cavities and the 

 general body cavity in which they are suspended. The American palaeontologists 

 also pointed out that " it appears more in accordance with all that is now known in 

 regard to the general anatomical structure, and the arrangement of the reproductive 

 organs of the living types of the Crinoidea, to suppose that the opening usually 

 regarded as the anus in the Blastoids was really such, or, as Dr. White and Mr. 

 Billings maintain, both mouth and vent, than that it was an ovarian aperture." 

 Meek and Worthen also express their agreement with Dr. White and others " who 

 reject the opinion that any of the other openings in these fossils were ovarian 

 apertures." They did not, however, offer any positive suggestion with regard to the 

 nature of the hydrospires or of the peripheral summit-openings. Billings - writing in 

 the same year expressed himself as convinced that the pectinated rhombs of the 

 Cystids are respiratory organs. " In all the species in which they occur they seem to 

 be constructed on the same general plan, i.e. the interposition of an exceedingly thin 

 partition between the circumambient water and the fluid within the general cavity of 

 the body." Billings confirmed Rofe's account of the identity in structure between 

 the hydrospires of the Cystids and those of Codaster ; and he further showed :! that 

 the various forms then referred to Pentremites " exhibit a gradual passage, from those 

 with the hydrospires almost entirely exposed, through others, in which they are 

 crowded more and more under the arms, until at length they become altogether 

 internal." He described the hydrospire of Pentremites proper 4 as " an elongated 

 internal sac, one side of which is attached to the inside of the shell, while the side 

 opposite, or toward the central axis of the visceral cavity, is more or less deeply folded 



longitudinally The object of the folding is, of course, to confine this large 



amount of surface to a small space an arrangement which at once proves the 



function to be respiratory." He also pointed out 5 that Pentremites has ten hydro- 

 spires connected together in pairs, each pair communicating with the exterior by a 

 single interradial opening which he proposed to call a spiracle. 



Billings further endeavoured " to show the gradual passage or conversion of the 

 respiratory organs of the Cystidea, Blastoidea, and Palaeocrinoidea into the ambulacral 



1 Proc. Acad. Nat. Sci. Philad. 1809, p. 85. 



2 American Journ. Sci. 1869, vol. xlviii. pp. 75, 76 ; Ann. & Hag. Nat. Hist. 1870, vol. v. p. 257. 



3 Ibid. p. 79 ; id. p. 261. « Hid. p. 80 ; id. p. 263. 

 5 Ibid. 1S70, vol. xlix. p. 54 ; id. 412. 



