CiiAP. III. GENUS PLEUROBRACIIIA. 217 



vature as the outer surfiice. Thus every layer of the intcramhnlacral sydcm is doubly 

 convex, and therefore thickest along the median line of the lobe, and thins out nearly 

 to an edge, on each side, where it meets the ambulacral layer. It also thins out 

 at the actinal and abactinal ends, and finally terminates [Fig. 2-3 n^ it') on a line 

 with the ends of the rows of locomotive flappers. The longer axes of the cells of 

 this system trend more or less parallel with the surfiice of each Ixand {Fig. 21 

 n 11^) to which they severally belong, and directly across from one ambulacral row 

 to another {Fig. 23 n^ n^). Therefore, when they contract longitudinally they teud 

 to draw the rows of locomotive flappers closer to each other, and consequently 

 decrease the peripheric extent of the spherosome. This, we shall see hereafter, has 

 an important bearing upon the peristaltic movements of the body. 



A band of similar cells {Fig. 21 m^) — the oral sgstcm — encircles the mouth, the 

 longer cells trending parallel to the edge of the lips, so that, when the mouth is 

 open and rounded, they are parallel also to the longer axes of those which con- 

 stitute the interambulacral system. In fact, the boundaries of the two systems — 

 the oral and the interambulacral — merge one mto the other, at least at the cor- 

 ners of the mouth, but more faintly at right angles to these points. 



The amhnlacral system consists' of bands of cells {Fig. 26 x^ x^ x^), which are identi- 

 cal in their nature with those of the interambulacral layers, and are eight in number, 

 one of each underlying a row of flappers {u). The thickness of a band is equal 

 to the distance between the ambulacral tulie (?') and the surfiice of the body, and 

 its lateral expanse corresponds to the breadth of the row of flappers which it 

 underlies; although it cannot be said whether this system belongs exclusively to 

 the locomotive apparatus, or takes part in the peristaltic movements of the body 

 in common with the interambulacral system into which it so gradually passes. 

 The latter agency most unquestionably obtains in the area about the mouth and 

 on the opposite pole, where the different peripheric systems merge into each other, 

 and where neither the ambulacral tubes nor the flappers are present: but in the 

 ambidacral region, the specialization of these cells, for the purpose of locomotion, 

 no doubt, is predominant ; and, perhaps, as we shall presently see, some of them 

 are exclusively devoted to the flappers. This assertion will appear to be true upon 

 inspecting Fig. 2(3, in the region {v?-) from which the flappers {u) arise. The ridge 

 (w) upon which each flapper is based is a simj^le projection from the surface of 

 the body; but its cellular constituents {v?) are arranged in a peculiar manner, which 

 indicates, as we think, the particular and exclusive use to which they are appointed. 

 The longer axes of these cells each and all trend outwardly in the direction of 

 the base of the flapper; and, unless we misinterpret appearances, they may be 

 recognized in the flajiper itself, where then- outlines appear as longitudinal stria?. 

 In plain terms, we would say that it is our conviction that these cells are arranged 



VOL. lu. 28 



