40 



THE VOYAGE OF H.M.S. CHALLENGER. 



superficial layers being shifted half the breadth of a zone, so that the chamberlets of those 

 layers lie over the zonal septa of the intermediate layer, and the zonal septa of the 

 superficial layers over the annular galleries into which the cylindi-ical chamberlets of the 

 intermediate stratum open at either end, as already described. When, by this shifting, 

 the complex plan of growth has been fully established, all subsequent increase takes 

 place in accordance with it. — The successive stages of this transition through the " sub- 

 typical" to the " tyijical" form of Orbitolites complanata are diagrammatical] y represented 

 on a larger scale in fig. 5. 



:?'"'' 



Fio. 5. — Diagrammatic representation of the progressive development of the Simple type of Orbitolites into tlie 



most Complex. 



p, primordial chamber ; c, d, circnmamhient chamber ; to, m', m-, m', m*, chambers of successive zones of Orbitolites Tnarginalis, each 

 having its single annular canal, ac, and its radial stolon-passage, r ; d,d',(P, chambers of successive zones of Orbiloliles duplex, each 

 having its single annular canal ac, and its double radial stolon-passage r ; e, e/, chambers of intermediate (fossil) form of Orbitolites 

 cmfuplanata, each having a pair of annular canals ac, a'c', with an interposed septum i, i, and having its superficial portions, s, s,' 

 still in continuity with the median columns m, m ; f,f-,p, ^, chambers of the typical form of Orbitolites complanata, each having 

 its double anniUar canal, its median columnar portion m, separated from that of the next annulus by the interposed septum i, 

 traversed by oblique pores, which appear as marginal pores, mp, at the edge of the disk ; but the superficial chamberlets, s, s, and 

 s", ^, alternating in position with the median, and each of them communicating with the annular canals of two zones, as shown at 

 ac, aV. 



The morphological differentiation which thus shows itself in the sarcodic body of this 

 " complex" type is thus of the very simplest character, involving (so far as our means of 

 judgment extend) no functional differentiation whatever. Its first stage consists in a 

 partial splitting of the single annular stolon of each zone (PL V. fig. 2, c, c') throughout its 

 entire length, and the vertical separation of its two halves from each other ; their continuity 

 being maintained, however, at certain intervals, so that, when drawn apart, they still remain 

 connected by the cylindrical columns {d, d, fig. 14) which form the intermediate stratum. 

 When these interpolated columns are looked at, not as parts of the regiilar annular 

 system of sarcodic sub-segments, but as bands of adhesion between the two sarcodic semi- 

 lannuli that are di-awn out by their separation, the frequent irregularities that may be 

 remarked in their arrangement (p. 35) are at once understood as confirming this view of 

 their homology. Each of the separated semi-annuli carries with it its own row of half 



columns (fig. 2, a, a', h, h') ; and these form the superficial planes of sub-segments (fig. 



14, c, c), which are at first in continuity with the interpolated shafts. And the 



displacement of these in the subsequently-formed annuli constitutes the second stage 



of this differentiation. 



