R94 WATER SYSTEM. 



variable, according to the size of the individuals examined. The tentacles 

 of the Demosticha are capable of great expansion and contraction, reach- 

 ing far beyond the spines (PL V. f. 1-8 ; PL X f. i). All the tentacles 

 do not terminate in a sucking-disk, as in the Diadematidae and the Arbacia- 

 dae for instance, where the abactinal tentacles are pointed, and where we 

 find neither terminal calcareous ring nor rosette {PL II'. f. &" ; PL IV. f. 8 : 

 PL V.f. 1-3,6-8). The ambulacra! tubes are most sensitive and contract 

 at the least disturbance. In the regular Echini the suckers of the actinal 

 side, which are by far the most powerful, are the main organs of locomotion, 

 and are capable of adhering with considerable force to the surface of the 

 rocks. They are, however, by no means the only organs of locomotion, as 1 

 have already shown when speaking of the habits of Arbaeia, in which the 

 spines are also used for locomotion on level surfaces, though of course the 

 suckers alone are used when the sea-urchin climbs steep slopes, to which it 

 can cling very (irmly by the suction of the ambulacra! tubes, — so firmly that 

 considerable force is necessary to separate a sea-urchin from a surface to 

 which it has become attached. 



In all the regular Echini, except the Cidaridae and some of the Echino- 

 thuriae. the ambulacral feet do not extend over the whole of the buccal 

 membrane. We only find near the actinostome live pairs of large buccal 

 tentacles provided with suckers; they are not always circular, but frequently 

 lobed as in Arbaeia. In the Cidaridae the imbricated plates, first noticed by 

 Desmoulins in the continuation of the ambulacral system, are pierced for 

 suckers identical with those of the rest of the ambulacral zone, extending 

 unbroken to the actinostome as far as the buccal tentacles, which are not 

 different in size from the other ambulacral tentacles. In all Desmosticha, 

 except the Cidaridae and Echinothuriae, the so-called gills protrude through 

 cuts, more or less deep, in the actinal edge of the coronal plates, between 

 the coronal plates and the buccal membrane. In the Cidaridae these gills 

 are found in cuts of the buccal membrane itself, close to the actinostome. 

 (Miiller denies their existence, yet he has himself figured the cuts through 

 which the gills pass in a species of Cidaris.) 



In the Clypeastroids the ambulacral tubes are already more complicated 

 and quite different in the so-called petals from those of the rest of the ambu- 

 lacral system. We find below the petals ambulacral snekers, having much 

 the same structure as those of the Desmosticha, capable of considerable ex- 

 pansion and contraction, with very well developed sucking-disks [PL XI '.,/'. 



