ECHINODERMA — CEINOIDS. 
61 
along each arm anel arm-branch, in the form of a tube or 
water-vessel, shown in some of the diagrams exhibited. This 
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 tlie 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 
i’alaeozoic, Devonian, Carboniferous and I’erniian, Jurassic, 
Cretaceous, and Tertiary. The foreign specimens are under 
the same stratigraphical divisions, to which, - however, the 
Trias is added. Some larger British specimens are also in 
these AY all-cases, and AA'all-case 16 contains large slabs from 
both British and foreign localities. 
Most of the Britisli Lower Palaeozoic Crinoids consist 
of the varied series of forms from the AA^'enlock Limestone of 
Dudley. Here one may compare the specimens of Botry- 
ocriims with the restoration (Eig. 28), and may note how 
pinnules are gradually evolved from simply forked arms. 
Adjoining are Mastiyocnnus, with its long scourge-like arms, 
and Thenaroo-inus, 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. Herpetocrinus is 
a curious form in which the stem coiled round the cup when 
tlie animal was at rest or dead, so that the fossils look like 
ammonites. In Calceocrimts arms of one side increase in 
size while tlie 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. Cyathocrimis and Gissocrinus are simple 
types, Irom which Eiudlocriium is not far removed. By the 
union ot the arm-branches in such a form arose Crotalocrinus 
Gallery 
VIII. 
Table-eases 
32 - 31 . 
Wall-cases 
17 , 18 . 
Wall-case 
16 . 
Table-case 
32 . 
