II. ] INCIPIENT STRUCTURES. 49 



The struggle by the sole action of which such a form was 

 developed must indeed have been severe ! 



The sea-urchins (Echinus) present us also with struc- 

 tures the origin of which it seems impossible to explain 

 by the action of " Natural Selection" only. These lowly 

 animals belong to that group of the star-fish class (Echino- 

 dermata), the species of which possess generally spheroidal 

 bodies, built up of multitudinous calcareous plates, and con- 

 stitute the order Echinoidea. They are also popularly known 

 as sea-eggs. Utterly devoid of limbs, the locomotion of 

 these creatures is effected by means of rows of small tubu- 

 lar suckers (which protrude through pores in the calcareous 

 plates) and by moveable spines scattered over the body. 



b 'b I 



AN ECHINUS, OB SEA-URCHIN. 



(The spines removed from one-half.) 



Besides these spines and suckers, there are certain very 

 peculiar structures, termed " Pedicellarise." Each of these 

 consists of a long slender stalk, ending in three short limbs 

 or rather jaws the whole supported by a delicate in- 

 ternal skeleton. The three limbs (or jaws), which stait 

 from a common point at the of end the stalk, are in the 

 constant habit of opening and closing together again with 

 a snapping action, while the stalk itself sways a.bout The 



E 



