422 



ZOOLOGY 



Fi(i. 33S. — Diagram of spine 

 of Sea-urchin showing 

 mode of articulation. 

 m. muscle ; 6. ligament. 

 (Prom Leuckart.) 



arranged in ten zones, five ambulacral and five inter-ambulacral, 

 as described in the account of Echinus, with peristome, periproct, 

 ocular and genital plates, and madreporite. Spines (Fig. 338), 

 pcdiccllarice (Fig. 339), and sphceridia are present, as already 

 described (p. 394), the last-named appen- 

 dages, however, being absent in one group. 

 The spines are usually defensive organs 

 simply, but in some Sea-urchins they act 

 also as the locomotive organs, the animal 

 moving by their agency along the sea-bottom. 

 The tube-feet, which are arranged in a 

 double row in each ambulacral zone, are 

 extremely extensible, and terminate in suck- 

 ing-membranes strengthened by a calcareous 

 rosette. An unpaired tentacle, corresponding 

 to that of the Asteroidea, is supported on 

 each of the ocular plates at the ends of the 

 ambulacral zones. Two tube-feet in each 

 double row, situated on the peristome, are 

 likewise of the nature of tentacles, and are 

 sometimes devoid of sucking-membranes. 

 Corresponding to the dermal branchial of 

 the Asteroidea are, in the majority, five 

 pairs of branched, hollow appendages surrounding the peristome. 

 Surrounding the mouth are five teeth, supported by an elaborate 

 system of ossicles (Aristotle's lantern, see p. 397), and a ring of 

 processes, the auricles, from the interior of the corona, surrounds 

 this and gives attachment to some of the muscles by which the 

 ossicles are moved. 



In the heart-shaped forms or Heart-urchins (Fig. 340) the 

 corona is heart-shaped, the mouth is usually 

 more or less eccentrically placed on the oral 

 surface, and the peristome is usually trans- 

 versely elongated; the anus is on or near 

 the border beween the two surfaces. The 

 ambulacral areas do not run continuously, 

 but stop short at the margin (petaloid am- 

 bulacra) ; one of them, the anterior, is usually 

 unlike the others and frequently devoid of 

 pores. The genital and ocular plates are 

 in the middle of the aboral surface, where 

 the ambulacra converge, and are thus widely 

 separated from the anus ; there are usually only four genital plates, 

 and the genital apertures may be reduced to two. Slender spines 

 beset the entire surface and are the chief organs of locomotion. 

 Modified spines, the clamdai, surround the anus in a ring and are 

 distributed elsewhere. A few pedicellarisB are present in the 



IV:. 389.— Pedicellaria of 

 Arbacia punctulata. 



(From Leuckart.) 



