ECHINODEEMA— CEINOIDS. 



61 



along each arm and arm-branch, in the form of a tube or Gallery 

 water-vessel, shown in some of the diagrams exhibited. This VIII. 

 gives off side-branches to little thin-walled tentacles, which 

 serve as gills and as sense-organs. When touched they with- 

 draw and the groove is closed over by the little covering- 

 plates that are generally present. They are again extended ' 

 by the pressure of the water in the hydraulic system, and 

 this system is kept full of water by means of openings in 

 the covering or lid of the cup, through which water is swept 

 by minute vibrating lashes (cilia) ; frequently these openings 

 are confined to a single sieve-like plate, the madreporite. 

 One can understand how this system also was developed in 

 connection with a fixed mode of life. But its importance, 

 like that of the five-rayed symmetry, is due to the fact that 

 it is also found in the free- moving Echinoderms. For these 

 and other reasons it has been supposed that the chief char- 

 acters of the Echinoderms as we know them in modern seas 

 are due to their descent from a fixed ancestor. 



We may now pass on to the general series of fossil 

 Crinoids. The British specimens are grouped under Early Table-eases 

 Palaeozoic, Devonian, Carboniferous and Permian, Jurassic, 32-31. 

 Cretaceous, and Tertiary. The foreign specimens are under Wall-eases 

 the same stratigraphical divisions, to which, however, the ^'^^ 

 Trias is added. Some larger British specimens are also in 

 these Wall-cases, and Wall-case 16 contains large slabs from Wall-ease 

 both British and foreign localities. l^- 



Most of the British Lower Palaeozoic Crinoids consist Table-case 

 of the varied series of forms from the Wenlock Limestone of 

 Dudley. Here one may compare the specimens of Botry- 

 ocrinus with the restoration (Fig. 28), and may note how 

 pinnules are gradually evolved from simply forked arms. 

 Adjoining are Mastig ocrinus, with its long scourge-like arms, 

 and Thenar ocrinus, both with a large extension of the cup- 

 lid looking like a wicker-basket ; this is the ventral sac, 

 through which passed the end of the gut. Herpctocrinus is 

 a curious form in which the stem coiled round the cup when 

 the animal was at rest or dead, so that the fossils look like 

 ammonites. In Calceocrinus the arms of one side increase in 

 size while the others gradually disappear, so that 'the five- 

 rayed symmetry of the cup is also partly lost, and the crown 

 hangs down from the stem, looking like the head of some 

 large-billed bird. Cyathocrinus and Gissocrinus are simple 

 types, from which Enallocrinus is not far removed. By the 

 union of the arm-branches in such a form arose Crotalocrinus 



