218 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



PBRISOMIC PLATES. 



The perisomic plates as usually understood are the plates developed within, 

 and entirely confined to, the integument of the ventral surface. They vary very 

 considerably in size in different species, and even in different parts of the same 

 individual. Though in general their shape and manner of occurrence is fairly 

 uniform, it is sometimes quite fortuitous ; usually they are quite irregular, but in 

 places where the integument is more or less modified or assumes a special form, as 

 along the raised borders of the disk ambulacra or in the so-called lappets fringing 

 the sides of the ambulacral grooves of the arms and pinnules, the included calcareous 

 deposits are cast or molded during formation into definite shapes. This is espe- 

 cially noticeable in the case of the covering, or side and covering, plates which occur 

 along the brachial and pinnule ambulacra forming such a characteristic feature in 

 the Calometridse, Thalassometridse, Charitometridfe, and (covering plates only) 

 certain genera of the Capillasterinse. 



Originally the body of the crinoid was entirely inclosed within a calcareous 

 capsule consisting of a central plate (commonly represented by a stem), three 

 alternating rows of calyx plates of which the last bore processes on their outer 

 borders, and a circlet of five apical plates (orals) about the pole opposite the central 

 plate the individual elements of which agree in orientation with the elements of the 

 middle row of calyx plates. Such a condition is seen in the pentacrinoid young of 

 the comatulids after the formation of the arms. 



Development from this type has been through the reduction of the calyx plates 

 row by row and the resorption of the orals, so that a progressively increasing part 

 of the body wall of the animal was left unprotected by the original covering. 



In most of the comatulids this process has been carried so far (pai't 1, fig. 74, 

 p. 127) that the representatives of the original calyx plates now merely form a 

 small button beneath the center of the visceral mass, which rests mainly on the arm 

 bases, with its entire ventral and much of its lateral and dorsal surface exposed. 



As the original calyx plates withdraw, leaving areas of unprotected perisome, 

 new skeletal elements in the form of minute spicules appear in the latter, which 

 may persist as spicules of various shapes and sizes, or may develop into more or 

 less perfected plates. 



The first vestiges of the calyx plates (fig. 1200, pi. 31) are in all ways similar 

 to the spicules in the perisome, but while the latter ordinarily do not develop, or 

 develop irregularly, the former grow very rapidly and regularly into definite cal- 

 careous structures (figs. 1200, 1197, 1196, pi. 31). The fact that the perisomic 

 spicules sometimes develop rapidly and regularly into very definite and highly 

 complicated plates, as in the case of the side plates on the pinnules of the species of 

 Calometridse (figs. 835-842, p. 405), Thalassometridee (figs. 859-86G, p. 419), and 

 CharitometridsB (fig. 816, p. 385), shows that there is no tangible fundamental 

 diiference between the primary and the perisomic plates. 



The perisomic plates, ontogenetically similar to the primary plates, and wher- 

 ever this is possible running the same developmental course, must therefore be 



