274 OUTLINES OF EVOLUTIONARY BIOLOGY 



sea-urchins and sea-cucumbers. The subdivision to which it 

 belongs the Crinoidea is characteristically a deep-water group. 

 It is also a group of great antiquity, the remains of crinoids 

 such as the well-known " Encrinites " of the Carboniferous period 

 being abundant in certain Palaeozoic formations. 



Both the Palaeozoic crinoids and the surviving deep-sea mem- 

 bers of the group, such as Pentacrinus (Fig. 125), are stalked 

 forms, the " calyx," with its radiating arms, being attached to 



FIG. 123. FIG. 124. 



FlG. 123. The Feather Star (Antedon bi/ida), nat. size. 

 FlG. 124. Pentacrinoid Stage in the development of Antedon, X 14. 

 dr. cirri ; st . stalk. 



the end of a long, jointed, calcareous stem, the lower extremity of 

 which is permanently fixed to the sea-bottom. Antedon, and the 

 other shallow-water crinoids of the present day, on the other 

 hand (Fig. 123), have no stalks but in place thereof a number of 

 slender " cirri " whereby they temporarily attach themselves to 

 seaweed or other objects. 



Now in the course of its development from the egg the feather- 

 star passes first through a free-swimming larval stage and then 

 through a fixed stalked stage (Fig. 124), known, from its resem- 

 blance to the deep-sea genus Pentacrinus, as the pentacrinoid 



