512 



ECHINODERMATA ECHINOIDEA 



discriminated from the interambulacral by the presence of pores 

 to permit the passage of the tube -feet. These pores are 

 arranged in pairs, and each pair corresponds to a single tube- 

 foot, since the canal connecting the ampulla with the external 

 portion of the tube -foot is double in the Echinoidea. In 

 Echinus esculentus there are three pairs of such pores in each 

 plate, in Strongylocentrotus droebachiensis four pairs. The 

 ambulaeral plate is really made up of a series of " pore-plates," 

 -each carrying a single pair of pores, and these become united in 

 threes in Echinus and fours in Strongylocentrotus, while in primi- 



FiQ. 228.— The so-called 

 calyx and the peri- 

 proot of Echinus 

 esculentus. x 4. 1, 

 Genital plates with 

 genital pores ; 2, 

 ocular plates -with 

 pores for terminal 

 tentacles of the radial 

 water-vascular canals ; 

 3, madreporite ; 4, 

 periproct with irregu- 

 lar plates ; 5, anus. 

 (After Chadwick. ) 



tive forms like the Cidaridae they remain separate. Each 

 ambulacral and interambulacral area ends at the edge of the 

 periproct with a single plate. The plate terminating the 

 ambulacral band is pierced by a single pore for the exit of 

 the median tentacle, which, as in Asteroids, terminates the 

 radial water-vascular canal. Thus the aboral end of the radius 

 in an Echinoid corresponds to the tip of the arm in an Asteroid. 

 The plate is termed " ocular," because the terminal tentacle has 

 a mass of pigmented cells at its base ; but no eye-cups can 

 be seen, and there is no evidence that this spot is specially 

 sensitive to light. Species which show special sensitiveness 

 to light have often a large number of what we may perhaps 

 term secondary eyes. The plate terminating the interambulacral 



