216 



PHYLUM ECHINODERMATA. 



FIG. 155. Radial vessel from the lower part of an 

 ambulacrum of Clypeaster placunarius (after J. 

 Muller). a ambulacral canal ; b lateral branches 

 of the same ; c pores ; d ampullae. 



pore-areas the lateral branches of the radial canal are long and connected 

 with a number of ampullae and tube -feet (Fig. 155) ; in the petaloid 

 areas the branches which supply the respiratory tube-feet of the yoked 

 pores supply the locomotive feet as well (Fig. 156). In the petaloid regions 



the minute pores are arranged 

 in rows and either perforate 

 the plates or lie along the 

 sutures (the yoked pores be- 

 ing between the plates), while 

 beyond the petals the distri- 

 bution is irregular. The 

 small pores increase in num- 

 ber with age. 



In some genera with dif- 

 fused pore-areas (e.g. Arach- 

 noides) there are grooves (not 

 branched) on the lower side 

 of the shell ; they are, how- 

 ever, without pores. It is no 

 doubt the presence of these 

 grooves which has led system- 

 atists to place Arachnoides with the Scutellidae. In many Clypeastroids the 

 ambulacral plates have small internally projecting processes, which may 

 unite with one another to form a kind of internal shell separating a much 

 broken up space, in which the water- vascular canals and the ampullae of 

 the minute feet lie, from the general cavity of the shell. The petaloid 

 ambulacral plates are often sunk below the level of the rest of the test 

 and may serve as brood-cavities or marsupia. for the young. 



In the Cassidulidae, the lower ends of the ambulacra dilate into petals 

 with crowded tube-feet, arranged round the peristome. These peristomial 

 rosettes are called phyllodes (Fig. 173). The interradial marginal plates 

 of the peristome be- 

 tween the phyllodes 

 are slightly pro- 

 tuberant, and the 

 whole figure inter- 

 radial protuberances 

 and phyllodes is 

 called the ttoscelle. 



In the Exocyclica 

 neither the ambula- 

 cral nor the inter am- 

 bulacral plates are 

 continued on to the 

 peristomial mem- 

 brane, and in the 

 Clypeastroids and 

 Spatangoids the in- 

 terambulacral part 

 of the peristome is usually constituted of a single plate only. 



In the Spatangoida, in which the mouth is in front, the posterior inter- 

 radius is long. The part of it on the lower surface is often protuberant and 

 called the plastron. In those Spatangoida with a transversely elongated 



FIG. 156. Ambulacral plates and vessel from the petaloid region 

 of an old Clypeaster placunarius (after J. Muller). a radial 

 vessel ; b its lateral branches ; d ampullae of locomotive feet, 

 the fine pores of which are shown in the upper part of the figure ; 

 e ampullae of ambulacral gills. 



