698 WATER SYSTEM. 



owing to the absence of bourrelets. The radiating ambulacra] tubes send to 

 the pores forming the phyllodes much larger branches than to the pores of 

 the remainder of the ambulacra! system (PI XXIII. f. l ; PL XXXIII. f. i ; 

 PI. XXXIV. f. 4). 



In the odd anterior ambulacrum the ambulacra! tubes consist entirely of 

 so-called locomotive tentacles, that is, tentacles slightly rounded at the tip 

 without any prominent sucking-disk; when there is a peripetalous fasciole 

 present, we find, within the peripetalous fasciole. that the tentacles of the 

 upper part of the odd anterior ambulacrum are usually larger than those 

 nearer the ambitus, and that they are provided with a powerful indented 

 or scalloped sucking-disk, strengthened by radiating spokes extending to 

 the periphery. Whenever we find a subanal fasciole present, tentacles like 

 those near the actinostome protrude through the large pores of the field. 

 The Echinonidae are the only Petalosticha having ambulacral feet provided 

 with suckers extending from the buccal membrane to the abactinal pole. 

 They do not. however, extend on the buccal membrane itself; that seems 

 to be a feature eminently characteristic of Desmosticha, and is not found in 

 any of the Clypeastroids or Petalosticha thus far known. 



The tentacles between the phyllodes and the petals are usually simple 

 tubes, either pointed at the extremity or with rudimentary sucking-disks 

 ( PI XVII. f. n;). The tubes of the petals are lobed laterally as in the peta- 

 loid ambulacra of Clypeaster. In the Petalosticha the tentacles are not 

 the only organs of locomotion. These, except the tentacles of the phyl- 

 lodes, are, as is well known, too delicate to play more than a very second- 

 ary part in locomotion, so that in Spatangoids the spines of the actinal 

 side must be used to a considerable extent. In all Spatangoids the power 

 of locomotion is quite limited; the spathiform and spoon-shaped spines of 

 the actinal plastron, usually so much stouter and more powerful than those 

 of the rest of the test, are their main organs of locomotion. 



Another important structural distinction between the Clypeastroids and 

 the Spatangoids is found in the ambulacral system. In Spatangoids proper 

 the ambulacral plates of the abactinal part of the test alone are perforated 

 by two pairs of pores, while toward the ambitus and on the actinal surface, 

 where the ambulacral plates increase in size, they are pierced by a single 

 opening ; this, as I have already shown in Echinolampas, is due to the exten- 

 sion of one zone of the poriferous system towards the actinostome. while the 

 other extends no farther than the petals, or but little beyond them. In the 



