CLASS I CRINOIDEA OT 
apparatus is represented by an extremely thin-walled, finely perforated, con- 
voluted body, which occupied the vertical axis of the body cavity, and was 
contracted into a narrow tube toward the base (Fig. 252). 
in all recent Crinoids five (occasionally four) open ambulacral furrows lined 
wita 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 grooves runs an ambulacral vessel filled 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 ring canal. From the ring canal five short open tubes (stone or 
water canals) extend downwards into the body cavity and supply the ambulacral 
system with water. 
In the recent genera, Theumatocrinus, Rhizocrinus, Calamocrinus, Hyocrinus 
(Fig. 224), and in a large number of fossil Crinoids, a triangular oral plate is 
situated in each of the five angles of the mouth-opening. The apices of the 

Fig: 225. ‘ F 
i rae Fig. 226. Coccocrinus TOSACEUS, 
Lecythocrinus - Eiifelianus, : Roem. Devonian; 
Miiller. Crinoid with elon- Dorycrinus quinquelobus, Hall sp. Eifel. Calyx with ven- 
gated anal tube (after Specimen showing plates of the teg- tral pavement, twice en- 
Schultze). men and eccentric anus. larged (after Schultze). 
orals are directed towards one another, and between them run the ambulacra. 
The plates are extremely variable in size; and although well-developed in the 
larvae of Antedon and Pentacrinus, they become wholly resorbed before maturity. 
In a number of Palaeozoic Crinoids (Larviformia, Fig. 227) 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 
interambulacral plates (Fig. 224). In most of the Palaeozoic Camerata, and the 
recent Calamocrinus, the anus is placed at the upper end of a tube known as 
the 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 interambulacral 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 
