Mr. E. Billings on the Structure of the Crinoidea cOc. 409 



XLVII. — Notes on the Structure of the Crinoidea, Cystidea, 

 and Blastoidea. By E. Billings, F.G.S., Palajontologist 

 of the Geological Survey of Canada*. 



[Continued from p. 266.] 



5. On the Homologies of the Respiratory Organs of the Palceo- 

 zoic 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 disk of Cyathocrinus^ and also the in- 

 ternal ' convoluted plate ' of the palaeozoic Crinoids, with the 

 tubes radiating therefrom, belong to the respiratory and per- 

 haps, in part, to the circulatory systems — not to the digestive 

 system. The convoluted plate, with its tliickened border, 

 seems to foreshadow the ' oesophageal circular canal,' with a 

 pendent madreporic apparatus as in the Holothuridea " 

 {ante, p. 255, note.) I should have referred it to the ma- 

 dreporic system of the existing Echinodermata in general, 

 instead of to that of the Holothuridea in particular. At 

 the time the note was written I had in view the madreporic 

 sac of Holothuria, which, as will be shown further on, most 

 resembles in form that of Actinocrimis. The figures and 

 descriptions which follow are intended to show the gradual 

 passage or conversion of the respiratory organs of the Cystidea, 

 Blastoidea, and Palajocrinoidea into the arabulacral canal- 

 system of the recent Echinoderms, and that, as the convoluted 

 plates of the former have the same structure and connexions 

 as the madreporic sacs 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 Cry2)- 

 tocrinites, Malocystites, 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, Echinosphrvrites, Palceocystites, 

 and Protocystites, the whole of the external integument seems 

 to have been respiratory, as all or nearly all of the plates of 

 which it is composed are more or less occu]ued by variously 

 arranged poriferous or tubular structures. The Cystideans of 

 these tAVO groups hold the lowest rank of all those known. In 

 their general structure they are mere sacs, of a globular, 

 ovate, or (as in the case of Trochocystites) flattened form. 

 Their test consists of an indefinite number of plates without 

 any radiated arrangement. They were also, according to our 

 j)resent knowledge, the first to make their appearance, two of 

 * From Silliman'g xVmerican Journal of Science, January 1870. 



