viii ECHINODERMATA— MORPHOLOGY OF SKELETON 353 



two ambalacral ossicles and an adambulacral ossicle {cf. Fig. 310). In those 

 Asteroids which have four longitudinal rows of tube-feet, however, these apertures, 

 at some distance from the month, alternate regularly in such a way that the 

 laterally placed aperture of one interstitium is followed by a more median aperture 

 in the next interstitium, the next again being lateral, and so on. The connecting 

 line between the apertures of one and the same side of an ambulacrum in this case 

 forms a zigzag, the angle of which is the more pointed the narrower the ambulacral 

 ossicle. The consequence of this is, that the tube-feet which stand in the corners of 

 the zigzag line appear arranged in two rows, that is, in the whole ambulacrum, in 

 four rows. 



The oral aperture, which always lies in the centre of the ventral 

 surface of the disc, and into which the ambulacral furrows of the arms 

 converge, is surrounded by a circle of firmly connected calcareous 

 pieces, the external edges of which are in immediate contact with the 

 ambulacral and adambulacral ossicles. This circle forms the oral 

 skeleton of the Asteroidea. It is extremely probable that its separate 

 pieces (which in the five-rayed forms number thirty, and in forms with 

 a greater number of rays are six times as numerous as the rays) are 

 merely the transformed and more firmly connected proximal ossicles 

 of the ambulacral and adambulacral rows. In this case, in each ray 

 or arm, the first two pairs of ambulacral and the first pair of adambu- 

 lacral plates of these rows (in Ctenodiscus, the first three ambulacral 

 and the first two adambulacral pairs of plates) would take part in the 

 formation of the oral skeleton. The oral skeleton is ambulacral (in 

 many C'ryptozonia) or adambulacral (in the Phanerozonia and some 

 Ori/fitozonia), according as the ambulacral or the adambulacral portions 

 of the circle project the further into the oral cavity. 



(b) The Interambulacral Skeleton. 



This comprises the ambitus, i.e. the whole surface of the body 

 between the oral (or ventral) and the apical (or dorsal) regions, on both 

 of which, however, interambulacral plates may be found. The inter- 

 ambulacral skeleton thus forms the lateral walls of the arms. The 

 pieces constituting it are called marginal plates, and are arranged in 

 each lateral wall in two rows, one above the other. The upper row 

 consists of the supramarginal (Fig. 309 sm) and the lower of the infra- 

 marginal (im) plates. It only rarely happens (e.g. in Luidia) that 

 the marginal plates agree in number and length with the ambulacral 

 ossicles. The marginal plates, which in the order Fhanerozonia are 

 large and well developed, become reduced in that of the Cryptozonia, 

 being difficult to distinguish externally. They may be altogether 

 wanting, or else represented merely by microscopically small rudi- 

 ments. The row of inframarginal plates may be separated from that 

 of the adambulacral ossicles by a row of small intermediate plates. In 

 the same way a row of small intermediate plates may be intercalated 

 between the two rows of marginal plates. 



VOL. II 2 a 



