494 THE AXATOMY OF INVERTEBRATED ANIMALS. 



and the radii into two, making a total of forty pieces. In 

 their normal position, it must be remembered that the alveoli 

 and teeth are interambulacral, while the radii and rotulae are 

 ambulacral. Besides the interalveolar muscles already de- 

 scribed, this complex apparatus has protractor muscles arising 

 from the interambulacral region of the oral edge of the coro- 

 na, and inserted into the upper part of the alveoli ; slender 

 oljlique muscles, with a similar origin, but itiserled into the 

 radii; transverse muscles connecting the radii together ; and 

 retractor muscles arising from the arches of the auriculae, and 

 inserted into the oral ends of the alveoli. 



A similar but less complex oral skeleton exists in most 

 Clypeastrolda (Fig. 144, A), but nothing of the kind has yet 

 been discovered in the Spatangoida. 



In the Eehlnidea^ the circular ambulacral vessel lies be- 

 tween the oesophagus and the alveoli, and is usually provided 

 with five sacculated polian vesicles. There is a single madre- 

 poric canal, membranous in EihUius^^ but calcareous in Cida- 

 rls, which extends nearly in the axis of the body from the 

 circular vessel to the madreporic tubercle. Five radial ves- 

 sels run up the middle of the inner surface of the ambulacral 

 plates, which they reach by passing from the circular canal, 

 outward, beneath the rotulte, when these exist ; next, down- 

 ward, external to the interalveolar muscles ; and then, out- 

 ward, through the arches of the auriculae ; these give off 

 branches on each side to the pedicels, the bases of which open 

 into large ambulacral vesicles. The circular ambulacral ves- 

 sel of the Spatangoida has no polian vesicles, and no vesic- 

 ular appendages ; in the Clypeasters there are many vesicu- 

 lar appendages, but no polian vesicles. In most Echinoida., 

 all the pedicels are expanded into sucking-disks at their ex- 

 tremities, and are here strengthened by a calcareous plate 

 or plates; but, in Echinocidaris and some other Echinoida^ 

 the pedicels of the oral portion of the ambulacra only have 

 this structure, while those of the apical portion are pecti- 

 nated, flattened, and gill-like. Again, in tlie heterogeneous 

 ambulacra of the Clypeastrolda and Spatangoida, the forms 

 of the pedicels vary much. Thus Muller distinguishes four 

 kinds of pedicels in the Spatangoida: simple and locomo- 

 tive pedicels, without any sucking-disk ; locomotive pedicels, 

 provided with terminal suckers, and containing a skeleton ; 

 tactile pedicels, with papillose expanded extremities ; and 

 gill-like pedicels, triangular, flattened, more or less pecti- 

 nated lamellre. Two or three of these kinds of feet may 



