296 PROF. p. M. DUNCAN S EEVISCOIS" OF THE 



plates ; this peristomial membrane surrounds the Stoma or true mouth and is 

 attached to the pyramids of the Jaws ; but in the edentate kinds, or Nodosto- 

 mata the inner edge is free. In the Gnathostomes the Jaws are internal and 

 the teeth project free in the Stoma. The edge of the peristome may be 

 grooved or incised or not, and in the first instance external hranchice are 

 present. Within the test, close to the margin of the peristome, are modi- 

 fications and fusions of the interradial and of the ambulacral plates which, 

 as ridges and processes, united or not, surround or underlie the Jaws and 

 give attachment to their muscles ; the structure is the Ferignathic Girdle, 

 formerly called " Auricles." The ridges are interradial, as in Cidaris ; the 

 processes are ambulacral. Leading from the gullet within the test, and 

 passing through the j^y^C'^nids of the Jaws and upwards over the inner 

 regions of the ambulacra, are five variably developed finger-shaped organs, 

 in some genera — " Stewart's Organs," or Internal branchics. External 

 branehias occur in some orders which are thus ectobranchiata. The ordei's 

 without them are anectobranchiata. 



The ambulacral 'plates are always perforated by canals (one, two, or many) 

 ending externally in pores ; the interradia are more or less perforated in 

 the Clypeastroida ; the water-system from within becomes connected with 

 Tentacles or Pedicels which are placed outside the test, their bases covering 

 •dipore or a pair. The surface of all of the plates of the test is tuberculate 

 -y or granular, and may have epistroma ; the tubercles carry spines, some 



minute ones, and some granules support the stalks of pedicellaries ; sphe- 

 ridia occur in the actinal part of the ambulacra. They are usually visible, 

 but may be hidden. 



Plates. — Tests may be rigid or more or less flexible. Plates are more or less geo- 

 metrical, calcareous pieces forming the interradia, ambulacra, dorso-central 

 system, perignathic girdle, and are found upon the peristomial and peri- 

 proctal membranes ; they are named according to those parts of the test, 

 and may be numerous or few. Composed of lax or close branching rods of 

 carbonate of lime and intermediate organic structure, of cleaving calcite in 

 fossils. A plate has an ijmer or visceral surface, an outer with tubercles, 

 granules, and epistroma : and edges, the more or less transverse of which are 

 dorsal and actinal or aboral and adoral ; the remaining edges are vertical 

 or aslant. The edges of plates are in contact with those of others around, 

 and the plane of junction, or union, is the Suture. The edges in contact 

 may be fused or inseparable, or in simple contact through the intervention 

 of a thin organic tissue ; or the edge of one plate may carry knobs which 

 fit into sockets upon the opposite edge (dowelling) ; or laonincs of one edge 

 dovetail into grooves in opposite edges. The outer part of the suture (the 

 line of suture) is often visible at the surface of the test between plates, and 

 may be a simple line, or a slight or deep groove, with or vnthout pits and 

 deep undermining hollows. Plates may be united by soft structures in which 

 there is no carbonate of lime, and then there is no suturing, but more or 

 less flexibility of the test or part of it. The plates of the peristomial and 

 periproctal membranes are thus separated. The sutures which are vertical 

 and pass down in the median line of the interradia and the ambulacra 

 are Median ; the sutures between the ambulacra and interradia are Amhu- 



