34 THE VOYAGE OF H.M.S. CHALLENGER. 



An indirect communication is thus established, not only among all the sub-segments of 

 the same annulus, but among those of all the annuli of each superficial layer. 



The intermediate stratum, which, as already stated, constitutes the principal part of 

 the thickness of the disk, is the distinguishing feature of this type of structure. When 

 laid open by a section taken parallel to either surface, the appearances it presents differ 

 according to the plane traversed by the section. For if this plane be that of the 

 concentric annular galleries that lie immediately beneath the superficial layer, the section 

 (PI. VI. fig. 4, g, g') lays open these galleries ; in the floor of every one of which is a 

 series of large rounded openings h, which are the summits of annular rows of nearly 

 cylindrical chamberlets that lie beneath the galleries. In sections taken beneath 

 these galleries, however, so as to pass either in or near the median plane of the disk, the 

 concentric arrangement seems to have altogether given place to the excentric or " engine- 

 turned" {i, i), the directions of the excentrics being opposite (as shown at k, k) in 

 successive planes. There is no change, however, in the concentric arrangement of the 

 rows of chamberlets ; what is diS"erent being merely the mode of communication between 

 them. These communications are in reality just what have been shown in fig. 3 (p. 13) 

 to be characteristic of the Orbitoliue type ; each chamberlet communicating both with 

 its own adjacent chamberlet, and also with the two chamberlets which alternate with 

 it in the annulus external to its own, by a pair of passages. Now the columnar 

 chamberlets forming the successive annuli of this intermediate stratum have vertical 

 successions of pairs of such communications ; but the two passages that form each pair, 

 instead of lying in the same plane, alternate with each other vertically, so that no 

 horizontal section can pass through both sets at once, — although it not unfrequently 

 happens, in consequence of a flexure in the disk, that diff'erent parts of the same sectional 

 plane show passages of opposite obliquities. And thus it comes to pass that each horizontal 

 section lays open a series of oblique galleries, formed by the one-sided communications 

 between the chamberlets of successive annuli ; and that in a section taken in a plane 

 either a little above or a little beneath, the direction of the obliquity is reversed. This 

 arrangement, again, is better understood by reference to the sarcodic body of the animal, 

 as seen in vertical section (PL V. fig. 14) ; for each of the cylindrical sub-segments of the 

 nearer zone (d, d) is seen to communicate with two sub-segments of the zone e behind it, 

 by two rows of stolon-processes ; those which pass from each of the two contiguous columns 

 in zone d towards the single column that alternates in position with them in zone e behind, 

 inclining towards each other, so as to enter that column nearly in the same vertical line, 

 though in diff'erent horizontal planes. By this arrangement each of the several pores 

 (PL VI. fig. 4, d, d', d") that form the vertical rows at the margin of the disk, instead of 

 opening, like the single pore of the simple type (fig. 2, p. 12), into both the chamberlets 

 of the last-formed annulus between which it lies, opens into only one of them,— 7the pores 

 of the same vertical series opening alternately into the chamberlets on either side. 



