THE CYSTWS 3263 



mouth on the upper side; while others, like Mesites, which resembles Agelecrinus 

 in the arrangement of its grooves, were attached, if at all, by only a small part of 

 the under side. Yet other cystids are definitely attached by well-developed stalks, 

 and have their bodies inclosed by a limited 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 respira- 

 tion 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 {Glyptosphcera} 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 mem- 

 brane, lining the interior of the test. These structures are called hydrospires 

 (water breathers), and somewhat resemble the cribriform organs found in some 

 deep-sea starfish of the present day ( Porcellanasfer) , figured later on. Structures 

 called hydrospires have also been described in such cystids as Caryocrinus and 

 Echinosphcera; but it is doubtful whether these actually subserved respiration, al- 

 though 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 L,ias at L,yme-Regis, have been 

 termed starstones. 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 

 incrusting 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 inclosing 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 



