In the vicinity are valuable deposits of crinoid limestone, a coarse white building stone which takes a good polish.
MYZOSTOMIDA, a remarkable group of small parasitic worms which live on crinoid echinoderms; they were first discovered by Leuckart in 1827.
- Diagram of a simple form of Crinoid, with five arms, each forking once; the one nearest the observer is removed to expose the tegmen of five orals.
This crinoid has only two circlets of plates in the cup, but the cup analysed in the adjoining diagram has in addition infrabasals and a centrale C.
Will be appreciated by comparing the structure of a simple crinoid with that of some other types.
A crinoid reduced to its simplest elements consists of three principal portions - (i.) a theca or test enclosing the viscera; (ii.) five arms stretching upwards or outwards from the theca, sometimes single, sometimes branching; (iii.) a stem stretching downwards from the theca and attaching it to the sea-floor (see fig.
In addition to these three or four circlets of plates, two other elements were once supposed essential to the ideal crinoid: the dorso-central and the oro-central.
Those plates are perhaps constant throughout sea-urchins and starfish (though it would puzzle any one to detect them in certain Silurian echinoids), and they may be traced in some of the fixed echinoderms; but there is no proof that they represent the radials of a simple crinoid, and there are certainly many cystids in which no such plates existed.
Although, in the extreme correlation of the radial food-grooves, nerves, watervessels, and so forth, with a radiate symmetry of the theca, such a type differs from the Cystidea, while in the possession of jointed processes from the radial plates, bearing the grooves and the various body-systems outwards from the theca, it differs from all other Echinoderms, nevertheless ancient forms are known which, if they are not themselves the actual links, suggest how the crinoid type may have been evolved from some of the more regular cystids.
The difficulty of rearing the larvae in an aquarium towards the close of the metamorphosis may account for the slight information available concerning the stages that immediately follow the embryonic. Another difficulty is due to the fact that the types studied, and especially the crinoid Antedon, are highly specialized, so that some of the embryonic features are not really primitive as regards the class, but only as regards each particular genus.
The crinoid type was differentiated by the extension of the food-grooves and associated organs along radial outgrowths from the theca itself.