FORAMINIFERA NUMMULITES, ETC. 445 



ening piece to each columnar segment. These additional pieces 

 are at first very short ; but with the growth of every new zone, 

 they commonly increase in length ; and this interpolated portion 

 comes to constitute the principal part of the thickness of the disk. 

 While this change is going on, another is taking place in the 

 position of the superficial portions of the segments which are 

 above and below the annular stolons ; for these, whilst at first 

 seeming to be mere continuations of the columns beneath, and 

 being connected (like them) with the stolons of their own zones 

 alone, are so displaced, in the course of two or three zones, as to 

 arch over the space between the zones (as shown in Fig. 208), 

 and to connect themselves with the stolon, not only of their own 

 zone, but of the next. It has been thought desirable to enter 

 into the foregoing detail, since a more striking instance could 

 scarcely be drawn from any department of Natural History, of 

 the wide range of variation that may occur within the limits of 

 one and the same species; and the Microscopist needs to be 

 specially put on his guard as to this point in respect to the lower 

 types of Animal, as to those of Vegetable life, since the determi- 

 nation of form seems to be far less precise among such, than it is 

 in the higher types. 1 



290. The type of Foraminiferous structure which has been 

 just described, is in many respects peculiar, and may be con- 

 sidered as verging towards the Sponges. It is obvious, from 

 what has been said of the extreme freedom with which the 

 several segments of the sarcode body communicate with each 

 other, that they form one whole, in a far greater degree than they 

 do in the ordinary composite Foraminifera, whose segments are 

 more completely separated, and are very commonly connected 

 only by a few very slender threads of sarcode. Indeed if we 

 were to imagine a discoidal mass of sarcode to be traversed by 

 a reticulated calcareous skeleton, somewhat resembling that open 

 areolar texture which forms the shell of the Echinida ( 312), and 

 this network were to possess somewhat of that regularity in the 

 disposition of its successively formed parts, which is presented to 

 us in the spines of that group (Fig. 237), we should have no 

 unapt representation of the calcareous skeleton of the Orbitolite, 

 and of its relation to the animal which it envelopes and protects. 

 Now there are certain Sponges which have a reticular skeleton 

 composed of mineral matter ( 296), differing from that of the 

 Orbitolite in little else than the want of the zonular arrangement 

 which marks successive epochs of growth ; and we shall see that 

 the constitution of the soft body is essentially the same in one 

 case as in the other. A remarkable connecting link between the 

 Orbitolite and certain Sponges, seems, in fact, to be presented 



1 For a full account of the Organization of the Orbitolite, and of the various conditions 

 under which it presents itself, see the Author's memoir upon that genus in " Philos. 

 Transact." 1856. 



