CRINOIDEA. 421 



grooves " — which ultimately coalesce to form five primary grooves, 

 which are continued across the ventral surface of the disc to the 

 mouth (fig. 286, a). These ambulacral grooves, in living forms, are 

 ciliated, and along them currents of water are kept up, by which 

 organic particles are conveyed to the mouth. In a number of types, 

 including numerous extinct and a few living forms, the ambulacral 

 furrows are covered in superficially by two, or, more rarely, by four 

 rows of alternating calcareous plates of small size. 



The upper or ventral surface of the visceral mass or disc is in 

 all living Crinoids, whether stalked or free, covered with a leathery 

 skin containing calcareous granules or plates, which are sometimes 

 scattered, but sometimes very closely placed ; and it exhibits the five 

 principal ambulacral grooves as generally open furrows passing from 

 the bases of the arms to the mouth (fig. 286, a). The mouth itself 

 is central in position, or, rarely, excentric (Actinometra), and may be 

 surrounded with five triangular " oral plates," which alternate with 

 the ambulacral grooves. In most Comatulce, the oral plates are 

 present only in the early stages of development (fig. 288), and dis- 

 appear in the adult ; though they are relatively large and well de- 

 veloped in Thaumatocrinus. Almost all the Secondary and Tertiary 

 Crinoids resembled the ordinary living forms in the possession of a 

 plated ventral perisome and open ambulacral grooves ; and hence 

 such forms have been grouped together by Dr P. H. Carpenter under 

 the name of " Neocrinoids." In all recent Crinoids, further, the 

 upper surface of the body exhibits the aperture of the anus, which 

 is generally excentric, though central in position in some Actinometrce, 

 and which is usually placed at the summit of a proboscidiform emin- 

 ence (fig. 286, a). 



On the other hand, in the so-called " Pabeocrinoids," embracing 

 under this name all the Crinoids of the Palaeozoic rocks, the upper 

 surface of the calyx rarely exhibits open ambulacral grooves, nor is 

 the mouth-opening generally exposed to view. On the contrary, the 

 oral aperture and food-grooves are more or less completely concealed 

 beneath a superficial plated covering, the structure of which varies in 

 different groups. In very many cases, as in the Actinocrinida, 

 Platycrinidce, Rhodocrinidce, &c, the ventral surface of the calyx is 

 roofed over by a flat or vaulted canopy of calcareous plates, which 

 are firmly united with one another, and completely conceal the sub- 

 jacent mouth-opening and ambulacral grooves. Five plates which 

 meet in the centre of this vault correspond to the " oral plates " of the 

 Neocrinoids. One of these is larger than the rest, and immediately 

 behind it the vault is perforated by a single, excentric, or, rarely, 

 central aperture, which is often prolonged into a tubular " proboscis," 

 and which is to be regarded as the anus. In all such cases, the 

 ambulacral grooves are continued beneath the above-mentioned 



