272 



THE STELLEROIDEA 



without lateral pits and processes (Fig. XIV.) ; while in the Clado- 

 phiurae the ossicles articulate by hour-glass shaped surfaces (Fig. 

 XXIX.), and the four external shields are replaced by a soft, 

 granular integument ; in this group the arms are therefore capable 

 of movement in any direction, the arms frequently being able to 

 coil round any support, as in Astrosckema. A further change is 



FMJ. XXIX. 

 Articular surfaces of vertebral ossicles of A*lm*i-liriint. 



introduced by the branching of the arms ; this occurs in two 

 groups of the Cladophiurae, viz. in the Trichasteridae, in which 

 the arms branch a few times at their free ends, and in the Gorgono- 

 cephalidae, in which the arms branch repeatedly (Fig. XXXII.). 



The Oral Skeleton of most Ophiuroids is on the same plan as 

 that of Ophiura; the details however differ considerably. The 

 nature of the parts is shown very clearly in some Palaeozoic 

 genera, such as Stnrtzura. The apparatus here consists of a 

 syngnath, apparently composed of the plates of only a single arm 

 segment ; one of the plates of the central 

 vertebral pair is elongated to a bar, and forms 

 the mouth-frame, which is therefore clearly 

 ambulacral. The jaw plate attached to this 

 mouth-frame is the adambulacral plate of the 

 same segment, while the teeth upon this are 

 homologous with spines. 



In many recent Ophiuroids there is an 

 element in the armature in addition to those 

 of Ophiura ; this is the set of " dental papillae " 

 which are situated at the oral ends of the jaws 

 and project toward the mouth above the 

 teeth. 



I" Ophiura and its allies there are no 

 Pedicellariae, but a very primitive type of 

 ., them occurs in tne Cladophiurae, as in Tri- 



and retractor 'muscles, chaster (Fig. XXX.). 



Turning to the internal anatomy we find the 



most important changes in connection with the Water -vascular 

 System. In many genera of the Cladophiurae there is more than one 

 madeprorite, of which there may be one in each interradius. In 



FIO xxx 



Pediceiiaria of 'in- 



