104 



the ventral disc of Cyathocrmus Sind, Siho, 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 fore-shadow the * oesophageal circular canal ' with a pendant 

 madreporic apparatus as in the Holothuridea." I should have referred it 

 it to the madreporic system of the existing Echincdermata in general, 

 instead of to that of the Holothuridea in particular. 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 Actinocrinus. 

 The figures and descriptions, which follow, are intended to show the grad- 

 ual passage or conversion of the respiratory organs of the Cystidea^ Bias- 

 toidea and Palceocnnoidea into the ambulacral canal system of the recent 

 €cliinoderms, 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 Cryptocrinites, 

 MahcystiteSj TrocJiocystiteSj and apparently some others, whose test is 

 totally destitute of respiratory pores, being composed of simple, sohd 

 plates like those of the ordinary Crinoidea. In a second group of genera, 

 among which may be enumerated Caryocystites^ Echinosplicerites^ 

 Pal/eocystites <^nd 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 occupied by variously arranged, porifer- 

 ous or tubular structures. The Cystideans of these two groups hold the 

 lowest rank of all those known. In their general structure they are mere 

 sacks of a globular, ovate or, (as incase of Trocliocystites) flattened form. 

 Their test consists of an indefinite number of plates without any radiated 



Fig. 64, Fig. 65, 



