CRINOIDEA. 287 



open into the body-cavity. These are the representatives of the 

 stone-canal of other types. They are usually very numerous 

 and are without calcareous deposits in their walls. They open 

 into the general body-cavity, the portion corresponding to the 

 axial sinus of other types having become continuous in the 

 adult with the perivisceral space. The calycine pores, each of 

 which represents a madreporite of other Echinoderms, also open 

 as we have seen into the body-cavity. 



The radial canals give off lateral branches, each of which, in 

 Antedon, supplies three tube-feet. In correspondence with this, 

 the tube-feet are placed in groups of three at the sides of the 

 ambulacral grooves and are without ampullae. They are to be 

 regarded as purely sensory and respiratory structures, and are 

 often called tentacles. 



The water-pores are lined by a ciliated epithelium. In 

 Rhizocrinus lofotensis there are only five, one in each interradius, 

 and they open into the perivisceral cavity close to the opening 

 of the stone-canals which are also five in number. In other 

 Crinoids * the number of water-pores is very numerous (sometimes 

 over 100), and it has not been shown that there is any relation 

 between their openings and those of the numerous stone-canals. 

 They are placed on the interambulacral portions of the calyx- 

 cover, and they perforate the interambulacral plates if such are 

 present. In Actinometra they have been observed on some of 

 the proximal pinnules as well as on the calyx. 



The axial organ (genital stolon) occupies the axial portion of 

 the perivisceral cavity. Apically it has the form of a thin strand 

 in the axis of the chambered organ ; from this point it ascends 

 in the body-cavity, where it widens. Its oral end is narrowed 

 again to a few strands which are continuous (see below) with the 

 generative rachis in the arms. It consists of convoluted canals 

 lined by columnar epithelium and embedded in connective 

 tissue of the vascular modification. The canals anastomose 

 and end without leading to any organ. 



The vascular system is present and has the usual form of anas- 

 tomosing spaces in the mesoderm. It is richly developed in 

 the wall of the alimentary canal, over the axial organ, in a ring 

 round the mouth, and round the genital rachis and the genital 



* In Cya.thocrinus and other Fistulata there is said to have been a 

 multiporous madreporite. 



