296 ECHINODERMS. 



number of plates arranged in regular order. Some of these present a six- 

 rayed symmetry, such as Caryocrinus, while others are governed by a five-rayed 

 symmetry, such as Lepadocrinus and Porocrinus. Both of these groups have as 

 a rule better developed arms, which sometimes branch, and are usually five or 

 six in number according to the symmetry of the cup. Hence these forms are 

 much more like the crinoids than are the other cystids. 



In other Echinoderms the rays with their numerous tube-feet help the 

 respiration of the animal, but these were absent or very slightly developed in the 

 cystids. There are, however, other structures that are supposed to have served 

 the same purpose. In some (Aristocystis) the plates of the test are pierced by 

 simple pores, while in others {Glyptosphoirci) these pores are in pairs ; but in either 

 case the pores are scattered irregularly over the body, and possibly gave passage to 

 minute tube-feet. The development of these and their concentration in certain 

 areas of the test would produce an arrangement not unlike that of other 

 Echinoderms. Other cystids have certain portions of the test pierced by slits 

 {Lepadocrinus), and it seems probable that these permitted the surrounding water 

 to pass in to the membrane, lining the interior of the test. These structures are 

 called hydrospires (water-breathers), and somewhat resemble the cribriform organs 

 found in some deep-sea star-fish of the present day (Porcellanaster), figured later 

 on. Structures called hydrospires have also been described in such cystids as 

 Caryocrinus and Echinosphcera ; but it is doubtful whether these actually sub- 

 served respiration, although the true hydrospires may have been evolved from 

 some such undeveloped structures. 



Another point of interest in the cystids is the light they throw on the origin 

 of the crinoid stem, which is formed of a series of flat rings. The simple round 

 plates, with a circular hole through the middle, are often called St. Cuthbert's beads, 

 while those marked with five petals, so common in the Lias at Lyme Regis, have 

 been termed star-stones. Technically the two kinds are distinguished as Entrochi 

 and Astroites. They are familiar in the polished slabs of Mountain Limestone, 

 in which it may be seen how the long stem is formed of a number of these round 

 ossicles jointed together, and pierced throughout by a narrow canal. The ossicles 

 are joined by ligaments passing right through their solid substance, and endued 

 with slight muscular power ; the central or axial canal serving for the passage of 

 blood-vessels, which are surrounded by a sheath of nervous tissue that controls 

 the movements of the stem. By one end the stem is attached to the sea-floor, 

 either by a flattish encrusting extension of its calcareous substance, or by a number 

 of fine branches or rootlets, as in the root-crinoid (Rhizocrinus), herewith figured. 

 By the other end the stem is attached to the plates forming the cup enclosing 

 the body of the animal, and it is at this end that it grows, by the constant 

 development of new ossicles between the cup and the upper segments of the 

 stem. Now, if we suppose that the crinoids, like other Echinoderms, sprang from 

 sac-like ancestors with a number of irregular and small plates, it is difficult to 

 understand how such a stem was evolved ; but the mystery is elucidated by some 

 of the cystids and older crinoids. First, it may be noted, that in those cystids 

 possessing a crinoid-like stem, as well as in many of the older crinoids, the axial 

 canal of the stem is much larger than it is in later forms. Secondly, that in many 



