56 THE GENESIS OF SPECIES. [CHAP. 



action of " Natural Selection " only. These lowly animals 

 belong to that group of the star-fish class (Echinodermata), 

 the species of which possess generally spheroidal bodies, 

 built up of multitudinous calcareous plates, and constitute 



AN ECHINUS, OK SEA-TJRCUIN. 



(The spines removed from one-half.) 



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 tubular 

 suckers (which protrude through pores in the calcareous 

 plates), and by movable spines scattered over the body. 



Besides these spines and suckers there are certain very 

 peculiar structures, termed " Pedicellariag." Each of these 

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

 or rather jaws the whole supported by a delicate inter- 

 nal skeleton. The three limbs (or jaws), which start from 

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

 habit of opening and closing together again with a snap- 

 ping action, while the stalk itself sways about. The utility 

 of these appendages is, even now, problematical. It may 

 be that they remove from the surface of the animal's body 

 foreign substances which would be prejudicial to it, and 



