IX 



PHYLUM ECHINODERMATA 



389 



FIG. 309. Diagram of spine 

 of Sea-Urchin showing 

 mode of articulation. 

 tn. muscle ; b. ligament. 

 (From Leuckart.) 



The tube-feet, which are arranged in a double row in each 

 ambulacra! zone, are extremely extensible, and terminate in suck- 

 ing-membranes strengthened by a calcareous rosette. An unpaired 

 tentacle, corresponding to those 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, 

 being devoid of sucking-membranes. Cor- 

 responding to the dermal branchiae 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. 366), 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 Hear.tr 

 urchins (Fig. 311) the corona is heart-shaped, 

 the mouth is usually more or less eccentri- 

 cally placed on the oral surface, and the peristome is usually trans- 

 versely 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 con- 

 verge, 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 clavulce, 

 surround the anus in a ring and are dis- 

 tributed elsewhere. A few pedicellarise 

 are present in the neighbourhood of the 

 mouth, and sphseridia also occur. A series 

 of tree-like dermal branchias surround the 

 peristome. The " lantern of Aristotle," with 

 its teeth, is not represented. 



In the Clypeastridea or Cake-urchins the whole corona (Fig. 312) 

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

 sometimes notched at the edges or pierced by fenestraB. The mouth 

 is in the middle of the flat or concave ventral surface, the anus 



FIG. 310. Pedicellaria of 

 Arbacia punctulata. 



(From Leuckart.) 



