352 COMPARATIVE ANATOMY chap. 



may be compared with vertebrae, and ai-e the principal pieces of the 

 skeleton, form a long roof over the ambulacral furrow, which opens 

 downwards. In a transverse section through the arm of an Asteroid 

 (Fig. 309) we see that the roof of tlie furrow invariably consists of 

 four skeletal pieces. Two of these pieces — the ambulacral ossicles 

 {am) — form the greater part of the roof. They lie symmetrically to 

 the median plane of the arm, and articulate with one another along 

 the ridge of the roof. The two other skeletal pieces — the adambu- 

 lacral ossicles {ail) — meet the diverging edges of the ambulacral 

 ossicles, and so lie at the edge of the furrow, or, in other words, at 

 the lower lateral edges of its skeletal roof. 



The general form of the ambulacral ossicles is that of transversely elongated 

 clasps. Thej?^ are arranged in two longitudinal rows in close proximity to one 

 another, and in this way form the roof, which arches over the groove along the whole 

 of its course, from the tip of the arm to the mouth. 



In the Euastcroidca (to which sub-class all recent forms belong) the anilnilacral 

 ossicles of the two rows are arranged in pairs, each ossicle on one side of the roof 



Fig. 310.— Scheme of the oral skeleton of the Asteroidea, from the inner side (after Ludwig). 

 or. Oral plate (odoiitophore) ; M], tirst lower transverse inustle of theiainbulacral furrow ; Mi, tlie 

 liiterradial muscle ; I-VI, first to sixth ambulacral ossicles ; 1-tJ, first to sixth adambulacral 

 ossicles ; a, b, c, d, e, f, apertures for the ampuUse of the tnhe-feet. 



corresponding with one on the other side. In the Palccastcroidca, on the contrary, 

 the ossicles alternate, at least in the middle part of the arm. 



The (smaller) adambulacral plates usually alternate regularly with the ambulacral 

 plates. 



"We must here emphasise the important fact that the ambulacral ossicles of the 

 Asteroidea lie much deeper than the skeletal pieces of the same name in the 

 Echinoidea. In the latter class they are quite superficial, the radial trunks of the 

 water vascular system, as well as the radial nerves and the spaces of the schizocoel, 

 are to be found on their inner side ; whereas, in the Asteroidea, these organs lie on 

 the outer side under the ambulacral roof. Of the whole ambulacral vascular system 

 only the ampuUffi lie on the inner sides of the ambulacral ossicles, i.e. that turned 

 towards the general body cavity. 



Between evei'y two consecutive ambulacral ossicles there is one (and never more 

 than one) aperture for the passage of a tube -foot. The number of ambulacral 

 ossicles in a row thus always corres])onds quite accurately with the number of the 

 tube-feet on the same side of the ambulacral furrow. 



Each aperture for the passage of a tube-foot normally lies in the corner between 



