CLASS I 



CIMNOIDKA 



127 



apparatus is represented by an extremely thin -walled, finely perforated, con- 

 voluted Unly, which occupied the vertical axis of the body cavity, and was 

 contracted into a narrow tube toward the base (Fig. 232). 



In all recent (Yinoids fiv<- (occasionally four) open ambulacra! furrows lined 

 with epithelium conduct from the mouth to the tips of the arms, remaining 

 either simple or subdividing as often as there are arms. Underneath the floor 

 of the grumes runs an ambulaeral vessel tilled with water ; and accompanying 

 this are the blood and vascular canals and a nervous cord. Distensible tentacles 

 pass out from alternate sides of the ambulacra, and the latter unite to form a 

 circumoral ////'/ ni/ni/. From the ring canal five short open tubes (stone or 

 Water ////'/>) extend downwards into the body cavity and supply the ambulacral 

 system with water. 



In the recent genera, T/iaumatocrinus, Ithizocrinus, Calamocrinus, Hyocrinus 

 ( Kig. L'L'l), and in a large number of fossil Crinoids, a triangular oral plate is 

 >ituated in each of the five angles of the mouth-opening. The apices of the 



:. 825. 



I.l-l-l/t,, : !: I'lll ,/,/.. 



Miiller. Ciin.iici wjth .-ion- 



Katrtl anal tube (after 

 Scluiltze) 



, iii riini^ i/ui iii/iii'Ii ilnix, Hall sp. 

 Specimen showing plates <>f the tei;- 

 inen ami eccentric anus. 



FIG. L'i'7. 



<"/"'.- rosaceits, 

 KIIIMU. Devonian ; 

 Eifel. Calyx witli ven- 

 tral pavement, twice en- 

 larged (after Schultze). 



nrals are diivctrd towards one another, and between them run the ambulacra. 

 The plates arc extremely \ariable in size; and although well-developed in the 

 larvae of Jutn/nn and I'nihi.-rinus, they become wholly resorbed before maturity. 

 In a number of Palaeozoic Crinoids (Larmfarmia, Fig. 227) the summit is 

 entirely <>r 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 ambulacra] furrows being covered with more or less regularly arranged 

 inttrambuiacral plates (Fig. 224). In most of the Palaeozoic Camerata, and the 

 recent Calanworinus, the anus is placed at the upper end of a tube known as 

 the anal ////* or profacis. In the Fistulata, however, the anal opening is situated 

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



<>t the interaml.iilacral plates a greater or smaller number (in Calamocrinus 

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

 admission of water into the inner cavity. Pores evidently performing a 

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



