180 



INVERTEBRATE ZOOLOGY 



bers of this class. Of present-day forms, most are restricted to 

 the greater depths of the ocean. Though distinctly local in their 

 distribution, they occur in great numbers, as must have also 

 been the case in past geological times when they were abundant 

 enough to form beds of rock of considerable thickness. The 

 joints of the stems are very conspicuous in many limestone 



deposits. 



The calyx is usually a globular or 

 cup-shaped capsule which holds the 

 more important internal organs. 

 This cup is formed of two or more 

 circles of plates. The ring of plates 

 next to the point of attachment to the 

 stalk or column and extending upward 

 to the projections of the arms com- 

 prises the base of the calyx, within 

 which there may be either a single 

 circle of plates called the basals or two 

 circles. In this last instance, the 

 plates next to the stalk are termed 

 the infrabasals, while the others are 

 called the basals. A series of plates 

 designated as the radials follows the 

 cycle of basal plates. An arm has its 

 origin with each radial plate. The 

 arms are formed of a series of plates 

 Fig. 88.— Fixed larva of a continuous with the radials and may 



cv'inoid, Antedon rosacea. {After i ..i ■ ■, ^ ^ ^ -r 



Carpenter). "^ either Simple or branched. In 



some of the more highly organized 

 fossil forms, and in all of the recent crinoids, the arms are 

 furnished with pinnules alternating on opposite sides. In these, 

 the gonads are borne. Arms and pinnules are traversed on the 

 ventral surface by an ambulacral groove at the bottom of which 

 there is a tubular extension of the coelom. 



That part of the calyx which lies between the bases of the arms 

 may be either in the form of a membrane with thin calcareous 

 ossicles embedded in it or in the form of a series of plates making a 

 continuous disc. In or near the center of this area, the mouth 

 opening (Fig. 89) is plainly discernible, while in an eccentric 

 interradial position the anus usually occurs. The grooves 

 mentioned in the description of the arms continue across the 



