426 THE CANADIAN NATURALIST. [DeC. 



NOTES ON THE STRUCTURE OF THE CRINOIDEA, 

 CYSTIDEA, AND BLASTOIDEA. 



By E. BiLLiXGS, F. G S., Palfeoutologist of the Geological Survey 

 of Canada. 



(Reprinted from the American Journal of Science, II., vol. xlix, p. 51, and 

 continued from this vol., ante p. 293.) 



5. On che Homologies of the Respiratory Organs of the Palceozoic 

 and recent Echinoderms, and on the " Convoluted Plate" of 

 the Crinoidea. 



In a former note I have advanced the opinion that : — "The 

 grooves on the ventral disc of Cyathocrinus, and also the internal 

 '■'■convoluted plate'' of the Palaeozoic Crinoids, with the tubes 

 radiating therefrom, belong to the respiratory, and perhaps in part 

 to the circulatory systems — not to the digestive system. The 

 convoluted plate with its thickened border seems to foreshadow 

 the " oesophageal circular canal," with a pendant madreporic 

 apparatus, as in the Holothuridea." (This vol. ante, p. 282.) 

 I should have referred it to the madreporic system of the existing 

 Echinodermata in general, instead of to that of the Holothuridea 

 in p:n-ticul;n-. At the time the note was written I had in view 

 the madreporic sack of Holothuria which, as will be shown 

 further on, most resembles in form that of Actinocrlnus. The 

 figures and descriptions, which follow, are intended to show the 

 gradual passage or conversion of the respiratory orgaas of the 

 Cystideit, Blastoidea and Palaiocrinoidea into the ambulacral 

 canal system of the recent echinoderms, and that as the convoluted 

 plates of the former have the same structure and connections as 

 the madreporic sacks and tubes or sand canals of the latter, they 

 are, most probably, all the homologues of each other. 



Among the Cystideans we find several genera, such as Crypto- 

 criiiites, Milocystites, Trochocystites, and apparently some others, 

 whose test is totally destitute of respiratory pores, being composed 

 of simple, solid plates like those of the ordinary Crinoidea. In a 

 second group of genera, among which may be enumerated 

 Caryocystites, Echinosphoerites, Palceocystites, and ProfocystitfS, 



