22 SMITHSONIAN INSTITUTION 



Anus (As). The excretory orifice of the rectum, intestine, or hind gut. ' It is interradial 

 in position ; its opening may be central, excentric, marginal, or lateral ; placed at the distal end 

 of a central or sub-central tube, directly through the disk, or through the dorsal cup at the 

 angle between basal and radials ; it is located at all intermediate positions between these 

 extremes, according to growth of the gut. The anal tube is subject to singular modifications 

 in size and shape, from a small conical protuberance to a large club- or balloon-shaped appen- 

 dage, often far exceeding the calyx in size. The latter is a marked feature of certain Fistulate 

 crinoids, in which the anal opening often appears at the anterior side, and may be located at 

 the base of the tube, midway toward its summit, or at the end of a lateral spout. 1 These facts 

 have given rise to the impression that the appendage in such forms may have had other than 

 anal functions, and it has therefore been called the ventral sac, as distinguished from a mere 

 tube. Later investigation shows that this supposition is unnecessary, as all forms of tube in 

 which the anus is not at the distal end may be explained by the recurving of the gut toward 

 the anterior side, redoubling upon itself more or less completely, so that the opening may 

 emerge from the wall of the tube at any point between the distal end and the base. 



Orals (O). The five large interradially situated plates which surround or cover the 

 mouth. They are said to be symmetrical when of nearly the same size and form ; asymmetrical 

 when the posterior plate is pushed in between, or is larger than, the other four. In some 

 forms permanently, and in some temporarily, they occupy the entire ventral surface of the 

 calyx ; in others only a comparatively small space in the middle of the tegmen ; or they may be 

 completely resorbed in the mature individual. Some or all of the orals may be perforated, 

 serving the purpose of a madreporite. 



Perisome. The pliant integument, whether naked skin or studded with undifferentiated 

 plates or spicules, which covers the ventral side of the calyx in the Flexibilia and in the 

 Mesozoic and Recent crinoids, surrounding the mouth, supporting the food-grooves, and 

 extending for varying distances down between the rays and their divisions. Morphologically 

 the perisomic plates include the interradials and interambulacral plates, and in its plated con- 

 dition the perisome is regarded as the morphologic equivalent of the solid vault of the 

 Camerata. In some forms of the Inadunata classed as the Fistulata, the perisome of the 

 posterior interradius of the disk is extended upward into a large sac lodging the enlarged gut 

 recurved and redoubled upon itself. 



The mouth occupies the center of radiation, and is either tegminal or subtegminal. If 

 tegminal, or exposed, as in the Flexibilia and Articulata, the opening is surrounded by the 

 orals and ends of the ambulacra, or when orals are absent by interambulacral plates which 

 form a lip around it.- If subtegminal, it is completely closed, as in the Camerata and Inadu- 

 nata, by orals, fixed ambulacrals, or interambulacral plates, which form a roof over it. The 

 ambulacra radiate from the mouth or oral center to the ends of the rays, following the ventral 

 furrows of arms and pinnules. When subtegminal, they enter the calyx through the arm 

 openings at the upper edge of the dorsal cup ; when tegminal, they follow the surface of the 

 disk. They contain the food-grooves, ambulacral and ovarian vessels, and the axial canal. 

 The food-groove forms the upper passage, and beneath it in order lie the subtentacular canal, 

 the genital canal, and the axial canal. The axial canal contains the axial nerve cords which 

 communicate with the chambered organ at the dorso-central basin of the calyx, from which 

 they also pass downward into the stem. In most of the Paleozoic crinoids permanently, and 

 in the Antedon larva temporarily, the axial canals of the calyx are mere grooves at the bottom 

 of the ventral furrow ; but in the adult Recent crinoids, and in a few Paleozoic forms, they 

 are separated from the furrow by a limestone partition, producing what is called a dorsal 

 canal. 



1 See North American Crinoidea Camerata, various figures on Plate VII. 



