476 THE ANATOMY OF INVERTEBRATED ANIMALS. 



The separate elements of which the skeleton is composed 

 may be divided into three groups: the ossiciila, wliich, joined 

 end to end and united by connective and muscular tissues, 

 constitute the chief framework of the body ; the spines^ at- 

 tached to the ossicula by ligamentous fibres at one end, and 

 free at the other; and the calcareous structures contained iu 

 the pediceUarlCi'. On the antambulacral wall of the body, 

 the ossicula are elongated rods of very unequal lengths, 

 united in such a manner as to leave polygonal, rounded, or 

 elongated meshes. The sides and roof of each ambulacral 

 groove, however, are bounded by two series of regularly-dis- 

 posed and similar ambulacral ossicles, wdiich lean against one 

 another in the middle line above, diverge so as to inclose the 

 ambulacral groove, and, at their outer ends, abut upon thick, 

 short adanibulacral ossicles, which lie at the sides of the 

 groove (Fig. 139, D). 



Between every two ambulacral ossicles in the same half of 

 the ambulacrum there is a canal, formed by the junction of 

 notches in the oral and distal faces of the two ossicles. Con- 

 sequently there is a half-pore on the oral, and another half- 

 pore on the distal, face of each ossicle. The half-pore on the 

 oral face is always internal in position to the half-pore on the 

 distal face, and, as the part of the ambulacral ossicle which lies 

 between the two is thin, the row of pores, though it is really 

 single and bent in a sharp zigzag, appears at first sight to be 

 double. The ducts, which connect the ambulacral vesicles 

 with the pedicels, traverse these pores; and the comparatively 

 large and very flexible and extensile pedicels are thus so 

 closely packed together, that they appear to form a double 

 row on each side of the middle of the ambulacrum. 



At the circumference of the oral disk, the ossicles of the 

 ambulacra, diminished in size, and closely united together, 

 form a pentagcm, the angles of which answer to the ends of 

 the ambulacral grooves, round the oesophagus. The con- 

 joined outer ends of the pair of ambulacral ossicles nearest 

 the mouth project on the oral face, outside the buccal mem- 

 brane, as five vertical crests, armed with strong spines, which 

 are beset with pedicellarife. In correspondence with these, 

 five falciform folds of the perisoma, more or less calcified, pro- 

 ject into the cavity of the body. They are interradial in 

 position, and extend up to the aboral wall. Their inner 

 edges are free, and look toward the stomach; with one of 

 them, the madreporic canal and the sinus which accompanies 

 it are closelv connected. 



