PHYLUM ECHINODERMATA 



395 



rounded bodies termed the sphceridia, which are perhaps, like the 

 pedicellariae, to be looked upon as modified spines : they contain 

 "ganglion-cells arid are apparently organs of special sense, having 

 possibly the function of detecting changes in the composition of 

 the water. 



Projecting from the surface among the spines all the way from 

 the peristome to the periproct will be observed five double rows of 

 tube-fed (Fig. 321), which in a living specimen will be found to 

 be capable of great extension. These are similar to the tube- 



FIG. 321. StrongylocentrotUS, entire animal with the tube-feet extended. (From Brehm'g 



Tierleben.) 



feet of the Starfish, and have similar functions : the sucker-like 

 extremity of each is supported by a perforated sieve-like plate of 

 calcareous matter. Each double row of tube-feet occupies a 

 meridional zone of the surface, termed the ambulacral area, 

 corresponding to the ambulacral groove of the Starfish : the inter- 

 mediate zones are termed the inter-ambulaeral areas. At the oral 

 end of each ambulacral area on the peristome (Fig. 320) is a pair 

 of appendages similar to tube-feet, but shorter, and termed tentacles. 

 Ten shrub-like appendages, the dermal branchiae, are situated in 



