IX 



PHYLUM ECHINODERMATA 



413 



Fia. 343. Diagram of 

 spine of Sea-urchin, show- 

 ing mode of articulation. 

 b. ligament ; m. muscle. 

 (From Leuckart.) 



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 podia or tube-feet, which are arranged in a double row in 

 each ambulacral zone, are extremely extensible, and terminate in 

 sucking -membranes strengthened by a cal- 

 careous rosette. An unpaired tentacle, corre- 

 sponding to that of the Asteroidea, is sup- 

 ported on each of the ocular plates at the ends 

 of the ambulacral zones. Two podia in each 

 double row, situated on the peristome, are 

 likewise of the nature of tentacles (oral or 

 buccal tentacles), and are sometimes devoid of 

 sucking-membranes. Corresponding to the 

 dermal branchice 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. 389), 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. 345) the 

 corona is heart-shaped, the mouth is usually more or less eccen- 

 trically placed on the oral surface, and the peristome is usually 

 transversely elongated ; the anus is on or near the border between 

 the two surfaces. The ambulacral areas do not run continuously, 

 but stop short at the margin (petaloid ambulacra) ; 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 sole or chief organs of locomotion. 

 Modified (ciliated) spines, the clavulce, arranged 

 in narrow bands or fascioles, are variously 

 distributed round the anus and elsewhere ; 

 but are sometimes entirely absent. A few pedicellarise are present 

 in the neighbourhood of the mouth, and sphseridia also occur. The 

 " lantern of Aristotle," with its teeth, is not represented. 



In the Clypeastridea or Cake-urchins the whole corona (Fig. 346) 

 is usually greatly compressed so as to assume the form of a disc, 



Fio. 344. Pedicellaria of 

 Arbacia punctulata. 



(From Leuckart.) 



