The crinoid type was differentiated by the extension of the food-grooves and associated organs along radial outgrowths from the theca itself.
Will be appreciated by comparing the structure of a simple crinoid with that of some other types.
These mountains consist of detached remnants of a sheet of quartz conglomerates, interbedded with sandstones, containing crinoid stems and obscure brachiopods.
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 the vicinity are valuable deposits of crinoid limestone, a coarse white building stone which takes a good polish.
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.
MYZOSTOMIDA, a remarkable group of small parasitic worms which live on crinoid echinoderms; they were first discovered by Leuckart in 1827.
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.
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.