TYPE ECIIINODERMA. 573 



latter the hydrocoel-pore lies to the left of the median line, while in the 

 bilateral Echinoids it is situated in the right anterior inter radius. Nor 

 can it be regarded as indicating a primitive adult symmetry, which, though 

 usually disguised, exists in all the Echinoderms, since, as stated, the forms 

 in which it is most pronounced are the most highly differentiated members 

 of their group. It should be mentioned that in the less differentiated 

 forms the radiality is disturbed by the arrangement of the plates bordering 

 upon the peristome, as well as by the unpaired madreporiform tubercle. 

 For if the rows of plates of each ambulacra! region be indicated alternately 

 a and &, proceeding contrary to the direction of the hands of a watch, so 

 that the posterior interambulacral region is bordered by the plates Ba and 

 Ab, then it will be found that the plates bordering the peristome in the 

 rows Ab, Ha, Ca, Db, and Ea are large and usually pierced by a double 

 pore, while those of the rows Aa, Bb, Cb, Da, and El are smaller and 

 pierced usually by only one pore. This arrangement does not, however, 

 necessarily point to any special plane of bilaterality, but is interesting on 

 account of its constant occurrence in both the bilateral and the more radial 

 forms, whence it furnishes a means of identifying the plane of bilaterality 

 in the latter. 



Projecting inwards from the inner surface of the test in 

 the neighborhood of the peristome are frequently to be found 

 calcareous plate- or pillar-like processes termed auricula 1 (Fig. 

 265, au), which may either be confined to the interambulacral 

 plates or occur also on the ambulacrals, uniting in some forms, 

 such as Strongylocentrotus, in pairs, so as to form arches 

 through which the radial hj'drocoel-canals and nerve-cords 

 pass. In the flattened disklike forms, such as Echinarachnius, 

 these pillars are much more numerous, extending from the 

 oral to the aboral surfaces of the test. Attached to the outer 

 surface of the test are numerous spines, each of which is 

 hollowed out at its base, the hollow fitting over the convexity 

 of a tubercle upon the test. This ball-and-socket articulation 

 allows of a free movement of the spines in any direction, a 

 movement which is effected by muscles extending from the 

 test to the base of each spine, and forming a sheath around 

 its base. The spines thus serve as efficient organs of locomo- 

 tion, usurping this function entirely in some forms, while in 

 others they are aided by the tube-feet. They also in some 

 forms serve as defensive structures, as in Diadema, where they 

 are long and slender and readily penetrate the skin of less- 

 protected animals, or in Asthenosoma , in which the larger 



