3°° 



ECHINODERMS. 



5tH 



least as many more of a closely allied genus, Actinometro, as well as three other 



less common genera, named Atelecrinus, Eudiocrinus, and Promachocrinus. They 



_^ =r ^_ __ ._ are far more numerous 



Hi at the present day than 

 j,jv 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 

 Holop us, 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 



FEATHER-STAR, CLINGING TO A TUBE OF SABELLA WORM 



(iiat. size). 



