DESIGN IN THE ORGANIC WORLD. 



447 



Fig. II. 



Central Portion of Animal 

 Body of Orbit oliles com- 

 planata. 



off from it ; successive rings, with constantly increasing numbers 

 of sub-segments, being in like manner 

 budded off around the outer border of 

 their predecessors, sometimes to the 

 number of loo. The shell, moulded 

 upon this composite body, thus ac- 

 quires the very regular discoidal form 

 shown in Fig. I. ; and its vertical 

 thickness usually increases from its 

 centre towards its circumference. A 

 vertical section of the disk (Fig. III., 

 2) shows that the chamberlets visible 

 on its two surfaces form two super- 

 ficial layers, which communicate with 

 continuous annular galleries that lie just beneath them (Fig. III., 

 3, d' , d"), every chamberlet, a, opening at each end into one of 

 these galleries; whilst the intermediate part of the disk is occupied 

 by columnar chamberlets (b, b), which open at either end into the 

 annular galleries, and are connected with each other by several 

 ranges of oblique passages {e, e,f,f). The passages proceeding 

 outwards from the last-formed ring, open on the margin of the 

 disk as pores arranged in more or less regular vertical series (Fig. 

 III., i) ; and these pores constitute the only means of communi- 

 cation between the complicated cavitary system of the disks, and 

 the surrounding waters from which the animal that inhabits them 

 draws its nutriment. The substance of this animal is apparently 

 altogether protoplasmic. Notwithstanding this complexity in the 

 structure of the disk, there is not the least trace of differentiation 

 in the contents of the several series of chamberlets. On the 

 contrary, there is every reason to believe that a continuous inter- 

 change must be always going on between the protoplasmic sub- 

 stance of the central and that of the peripheral parts of the disk ; 

 so that the nutriment taken in by the " pseudopodial " extensions 

 which the latter puts forth through the marginal pores, may be 

 diffused through the whole multiple series of sub-segments, of 

 which the body of this organism consists. This I characterized 

 as the "complex" type of Orbitolite structure. 



The minute disks picked out of shore-sands, however, were 



