ECHINODERMA. CRINOIDEA 137 



mouth ; from this ring tubes hang down and open into the 

 body-cavity, which communicates with the water of the 

 exterior by means of pores. In connection with the radial 

 vessels are tubular processes, the tentacles, which form a 

 row on each side of the food-grooves, and correspond with 

 the tube-feet of the star-fish, but do not function in 

 locomotion. In addition to the nervous system already 

 mentioned, there is another supplying the aboral elements 

 of the skeleton ; from a centre at the aboral pole of the 

 calyx nerve cords are given off, which pass through canals 

 in the plates of the calyx to the arms and pinnules, and 

 also into the stem when present. 



The stem (figs. 55, 57, a) in the crinoids is more or less 

 flexible, and is sometimes several feet in length. It con- 

 sists of a number of segments, known as columnals, which 

 may be in the form of circular or pentagonal (occasionally 

 square or elliptical) plates ; or they may be higher than 

 broad, forming cylinders; these columnals articulate by 

 their flat surfaces, which are often provided with radiating 

 striae or with ridges in the form of a rosette. Each 

 columnal is pierced at the centre by a canal which is 

 circular or pentagonal and contains a prolongation of the 

 aboral nervous system and vascular organ. From the stem 

 small branches known as cirri are sometimes given off; 

 these have a structure similar to that of the stem, and 

 are also pierced by a central canal. The columnals are 

 generally of different heights — larger plates being separated 

 by smaller ; the former are first developed, and the latter 

 subsequently introduced between them. Cirri are borne 

 on some of the larger columnals. The lower end of the 

 stem may taper, but often expands and branches, forming 

 a root-like structure which serves to fix the animal. 



The part of the calyx below the origin of the free arms 



