THE STELLEROIDEA 247 



The oral skeleton is described as either ambulacral or adam- 

 bulacral, according as the ambulacral or adambulacral plates 

 are the more prominent. Viguier, in his important memoir on 

 the skeleton of the Asteroids, pointed out the existence of these 

 two types, and based his classification upon this character. As we 

 have seen, in the genus Asterias the oral skeleton consists of a 

 solid ring, in which the ambulacral plates form five prominences, 

 while the adambulacral plates are small, and occur in the angles 

 between them. Such a mouth-structure is described by Viguier as 

 "ambulacral." In Pentaceros (Oreaster), on the other hand, the 

 ambulacral plates are inconspicuous, and the adambulacral plates 

 project into the buccal cavity and form the jaws. This type is 

 Viguier's "adambulacral mouth." In both cases above the adam- 

 bulacral plates is a basal, interbrachial, or oral plate which is 

 called by Viguier the odontophore, although it does not bear the 

 teeth ; its value is of secondary importance. 



The Pedicellariae of Asteroids are of four main types. The 

 simplest form consists of a row of pairs of small, sessile, opposajjle 

 spines; these are the "pseudo-pedicellariae." The members .o'f 

 the second set are " sessile." The next advance is the develop- 

 ment of a stalk ; of these pedunculate pedicellariae there are 

 two varieties: (1) the " forficiform," in which the two hooks are 

 attached to the nearest end of the basal plate nearest to them ; 

 (2) the " forcipiform," in which the two hooks cross one another 

 and are attached to the end of the basal plate furthest from them. 

 Perrier has used the pedicellariae as the basis of his classification 

 of the Asteroids, on the ground that they are the degenerate repre- 

 sentatives of organs once more important. Other authors, however, 

 regard them as modified and elaborated spines, in which case they 

 are of little taxonomic value. 



Another type of spines occurs as part of the structures known 

 as " paxillae." Each paxilla consists of a thick plate supporting a 

 number of short, calcareous pillars, the summit of each of which 

 is covered by a group of small spines. In some Phanerozonia, such 

 as Astropecten, the paxillae occupy almost the whole abactinal 

 surface of the Asteroid (Fig. VII.). 



The Water- Vascular System is on the same plan as in Asterias, 

 but there are the following modifications : In most starfish there 

 is a pair of Polian vesicles in each interradius ; they rise from the 

 circumoesophageal vessel beside the Tiedemann's body. The 

 radial branches and its podia are simplified by the absence of 

 ampullae, as in Brisinga, or by the podia ending in points instead 

 of suckers, as in the Astropectinidae. Suckers are improvised in 

 such pointed podia by a contraction of the walls below the end. 



The number of madreporites is very variable among the 

 Asteroids ; in most of those with many rays there are several 



