586 TRANSACTIONS OF SECTION D. 



made up of a number of separate compartments or chambers, communicating with 

 one another by one or more narrow passages, and disposed in some regularly 

 symmetrical plan. In some the arrangement is a flat spiral, like that of a watch 

 spring ; in others helicoid,like a snail's shell. In some the series of chambers may 

 form a straight or slightly curved line, or they may alternate on either aide of a 

 straight axis. There is great variety in the plans on which the shells may be built. 

 They differ, too, in texture ; some are transparent, and their walls are perforated 

 by multitudes of minute pores, setting the interior of the chambers in direct com- 

 munication with the outer world, while in others the walls are semi-opaque, 

 white, and glazed like porcelain, and such perforations are absent. The shells 

 are composed, for the most part, of carbonate of lime contained in an organic 

 ' chitinous ' matrix, but in many cases grains of sand are included in the walls. 



The planispiral chambered shells present such a close resemblance to the shell 

 of a Nautilus that for a long time, notwithstanding their diminutive size, many of 

 them were actually included in that genus, among the cephalopod moUusca. 

 As knowledge advanced the Cephalopoda were divided by D'Orbigny into two 

 groups : the Siphonifires, in which, as in Nautilus, the Ammonites and Spirula, 

 the chambers are in connection by a siphon ; and the Foraminiferes, in which they 

 communicate by pores. 



If instead of examining the empty shells left stranded on the shore we take 

 seaweed from .shore pools or from shallow water and separate the adherent 

 particles by means of a sieve, similar Foraminiferous shells will be found in 

 the sand which comes through, and these will usually contain the live animal,. 

 If glass slides are set in the vessel on the sand, overnight, some of the animala 

 will generally crawl on to them, and they may then be taken out and ex- 

 amined. About these active animals, springing from various points at the periphery 

 of the shell, are multitudes of slender threads, forming fanlike or sheaf-like 

 groups, by which the animal is attached to the substratum, and by which it moves. 

 They are composed of a clear hyaline substance — protoplasm — containing scattered 

 granules. If the animal is killed and the shell dissolved by a weak acid, no 

 organs, such as muscles, stomach, brain, and so forth, are found in the interior, but 

 the same granular protoplasm is found to fill the interior of all the chambers. As 

 in the Protozoa in general, all the elementary functions subserved by the organs 

 of other animals are performed by the undifferentiated protoplasm. 



It was not until 1835 that the simple character of the soft parts filling the 

 shells of Foraminifera was recognised by Dujardin. He pointed out that, far from 

 being allied to such highly organised beings as the cephalopod mollusca, they 

 belonged to the simplest forms of animal life, such as Ajnceba, and proposed the name 

 Ehizopoda, which is still in use, for the class containing them. 



For many years, however, the correctness of Dujardin's views was matter of 

 dispute. One of the first zoologists to recognise their truth and confirm them was 

 the distinguished Yorkshire naturalist. Professor Williamson, who in 1849 pub- 

 lished his memoir ' On the Structure of the Shell and Soft Animal of Polystomella 

 crisjja,'^ in which, for the first time, the internal structure and the relation 

 between the chambers were correctly described. 



In the specimens described by Williamson the shell of Polystomella has the 

 following structure. Externally it is a nearly biconvex shell, symmetrical 

 about a median plane, and with a keel-like projection at the margin. In 

 young specimens sharp points like those of a spur often project from the keel. 

 The chambers of which it is composed are arranged in a spiral. They are convex 

 towards the mouth, i.e., on their anterior faces, and concave in the opposite direc- 

 tion. Moreover, each is produced on either side into a process, or alar prolonga- 

 tion, projecting towards the axis about which the spiral turns, i.e., towards the 

 convex prominence at the centre of each face. Thus each chamber of an outer 

 whorl of the spiral is placed, as it were, astride of the next inner whorl, and 

 the last whorl of the spire completely hides all the previously formed cham- 

 bers from view. Careful examination of the anterior face of the terminal 



1 



Trans, Mic.roscop . Soc, vol. ii. 1849, p. 159. 



