CLASS III 



CRINOIDEA 



177 



In certain of the Recent stulked genera, as in Hyocrinus (Fig. 272), in 

 the young of all the Recent species, and in a large number of fossil Crinoids, 

 a triangulär oral plate is situated at each of the five angles of the mouth-open- 

 ing. The apices of the orals are directed towards one another, and between 

 them run the ambulacra. Oral plates are extremely variable in size ; and 

 although well-developed in the larvae of the Comatulids and in the young of 

 many of the stalked species, they become wholly resorbed before maturity. 

 In a number of Paleozoic Crinoids (Fig. 275) the summit is entirely or in 

 large part composed of five oral plates which may be either laterally in 

 contact or separated by furrows. More frequently, however, the orals occupy 

 only the angles of the mouth-opening, the remaining area between the 

 ambulacral furrows being covered with more or less regularly arranged 

 interamhulacral plates (Fig. 272). In most of the Paleozoic Camerata, and in all 

 the Recent species, the anus is placed at the upper end of a tube known as the 



Fio. 273. 



Lecytliocrinus eifeliatius 

 Müller. Crinoid with elon- 

 gated anal tube (alter 

 Schnitze). 



Fig. 274. 



Dorycrlnus quinquelohus (Hall). 

 Specimen showing plates of the teg- 

 nien and eccentric anus. 



Fio. 275. 



Coccocrinus rosaceus 

 Roem. Devonian ; 

 Bifel. Calyx with ven- 

 tral pavement. X 2/, 

 (after Schnitze). 



anal tube or proboscis. In the Fistulata, however, the anal opening is situated 

 along the anterior side of the ventral sac, or between the sac and' the mouth. 



Of the interamhulacral plates a greater or smaller number (in Calamocrinus 

 all in the vicinity of the mouth) are perforated by pores for the admission 

 of water into the body cavity. Pores evidently performing a respiratory 

 function occur in some of the Fistulata ; but these, instead of piercing the 

 body of the plates, enter only their outer angles. Other Fistulata have a 

 madreporite. 



The ambulacra are frequently lined along their sides by more or less 

 rounded covering plates which are capable of being folded down over them 

 so as to serve as a protection ; these covering plates are, at least in the 

 Recent forms, perisomic plates developed in the marginal ambulacral lappets ; 

 they may occur alone, as in the genera Ehizocrinus, N'emasier, etc., but they are 

 usually separated from the pinnulars or brachials by a second series of squarish 

 or oblong plates known as the side plates. Both series of plates occur every- 

 where along the ambulacral grooves, but they become irregulär and ill-defined 

 011 the disk. In the Paleozoic Taxocrinus (Fig. 276), the covering pieces are 

 VOL. I N 



