3 °° 
ECHINODERMS. 
least as many more of a closely allied genus, Actinometra, as well as three other 
less common genera, named Atelecrinus, Eudiocrinus, and Promachocrinus. They 
are far more numerous 
at the present day than 
the stalked crinoids, and 
occur in all parts of the 
world, but their head¬ 
quarters are in the 
Eastern Archipelago. 
There are a few 
crinoids that have 
diminished their stem, 
but have nevertheless 
remained attached, so 
that at last the cup has 
come to be fixed on the 
sea-floor without the 
intervention of a stem. 
Such a form is the 
stumpy and thick-set 
Holopus, which is among 
the greatest rarities in 
museums. It lives at 
depths of about a 
hundred fathoms in the 
Caribbean Sea. Similar 
forms occur in some of 
the shallow-water and 
reef deposits of Jurassic 
and Cretaceous age. 
Many of these have 
become a little unsym- 
metrical and bent over 
in one direction; which 
may, perhaps, be ac¬ 
counted for by their life 
on reefs, where food is 
brought to them by 
currents flowing only 
in one direction. 
Next to the stem, 
the most characteristic 
structures of a crinoid 
are its arms. Each arm starts from one of the five plates that form the uppermost 
circlet in the cup. The arms are said to be radial in position, and those plates from 
which they start are specially distinguished as the “ radials.” In many forms, such 
ROSY FEATHER-STAR, CLINGING TO A TUBE OF SABELLA WORM 
(nat. size). 
