Sea Urchins. 53 



CHAPTER X. 



SEA URCHINS AND SEA CUCUMBERS. 



CLASS III. Sea Urchins (Echinoidea). The globulin 



or heart-shaped sea-eggs, found along our sea coasts, 



are representatives of the next group of Echinoderms. 



In these the surface is covered with movably jointed 



spines, each of which shows on section a beautifully 



reticulated structure which varies in each species and 



the attached end of each spine is hollowed to fit on a 



tubercle (fig. 27, p. 48) on the hard shell or corona 



beneath, with which it thus forms a ball-and-socket 



joint. On removing the spines FIG. 31. 



the shell is found to consist of 



numerous flat angular plates, 



arranged in 20 meridional rows. 



This corona has the mouth at 



one pole, which in the living 



animal is undermost and the 



excretory orifice at the opposite 



extremity : and from mouth to The a P ical end of the she11 



J } ... . of Echinus esculentus. 



apex the Shell IS divided intO ambulacral plates, Winter- 

 ."..', ,, ambulacral plates, opposite 



ten meridional segments, five of i the dark space is the 



i i . / T . j mad reporiform plate, ^anal 



Which COnSlSt Of plates pierced opening, c ocular plktes, d 



with holes for the ambulacral ovarian plates. 

 feet, and five of imperforate plates ; these are placed 

 alternately, and each segment, perforated or imper- 

 forate, consists of two rows of plates (fig. 31, a, b). 



The mouth is surrounded by a soft area of skin 

 bearing modified spines, modified tube- feet and pedi- 



