Sea Urchins. 53 



CHAPTER X. 



SEA URCHINS AND SEA CUCUMBERS. 



CLASS III. Sea Urchins (Echinoidea). The globular 

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

 are representatives of the next gioup 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 beneath, with 

 which it thus forms a ball-and-socket joint On 

 removing the spines there is 

 found under them a shell com- FlG - si- 



posed of numerous flat angular 

 plates, arranged in meridional 

 rows. This shell has the mouth 

 at one pole, which in the living 

 animal is undermost and the ex- 

 cretory orifice at the opposite 



extremity ; and from mOUth tO The apical end of the shell 



'. -..,-. of Echinus esculentus. 



apex the Shell IS divided intO ten e anal opening, c ocular 

 j . ,. f , , plates, d ovarian plates, 



meridional Segments, five Of Which maclreporiform plate. 



consist of plates pierced with 

 holes for the ambulacral feet, and five of unperforated 

 plates ; these are placed alternately, and each segment, 

 perforated or unperforated, 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- 



