168 THE SEA SHORE 



of the five perforated plates may also be seen, as well as the 

 ovaries or egg-producing organs and the bases of the five jaws with 

 then: complicated system of muscles. 



A little acquaintance with the commonest of the British sea 

 urchins will show that they may be divided into two well-defined 

 groups, one containing the globular or subglobular forms, of which 

 the common sea urchin or sea egg (Echinus sphcera) above 

 described, is a type, as well as the pretty little Green Pea Urchin 

 (Echinocyamus pusillus), and the little Purple-tipped Urchin 

 (Echinus miliaris), which is found principally on the west coast 

 of Scotland ; while the second group is formed by the less 

 symmetrical Heart Urchins, which differ from the others in several 

 interesting particulars of structure and habit. 



FIG. 114. SEA URCHIN DISSECTED, SHOWING THE DIGESTIVE TUBE 



These heart urchins (Plate IV., fig. 4) are covered with short, 

 delicate spines which are not much used for purposes of locomo- 

 tion, the animals moving from place to place almost entirely by 

 means of their tube-feet, while the globular urchins travel princi- 

 pally by their spines, which are stouter and more freely moved on 

 well-formed ball-and-socket joints. Also, while in the globular 

 species the perforated plates that admit of the protrusion of the 

 feet are arranged with a perfect radiate symmetry, those of the 

 heart urchins are confined to one side of the shell ; and the digestive 

 tube, which in the former terminates in the pole opposite the mouth, 

 in the latter ends close to the mouth itself. Further, the heart 

 urchins do not possess any kind of dental apparatus. 



