402 



NA TURE 



[August i6, 1906 



soft parts filling the shells of Foraminifera was recognised 

 bv 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 Amoeba, and proposed the name Rhizopoda, 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 dis- 

 tinguished Yorkshire naturalist, Prof. Williamson, who in 

 1849 published his memoir " On the structure of the shell 

 and soft animal of rolystoinella crispa,"^ 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 

 direction. Moreover, each is produced on either side into 

 a process, or alar prolongation, 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 chambers from 

 view. Careful examination of the anterior face of the 

 terminal chamber reveals a row of foramina along the line 

 where the chamber, including its alar prolongations, rests 

 against the whorl which it bestrides. It results from 

 what has been said that they present a V-shaped line. 

 These foramina are the main openings by which the cavity 

 of the last chamber opens to the exterior. Each chamber 

 of which the shell is composed has been in its turn the 

 terminal chamber, and the openings which then led to the 

 exterior subsequently form communications leading from 

 chamber to chamber. As we trace them back to the earlier 

 chambers they become fewer in number until only a single 

 foramen is found between the chambers. In specimens of 

 the type we are considering a comparatively large globular 

 chamber is the starting point from which growth pro- 

 ceeded. A short passage leads to the second chamber, 

 which has a peculiar shape, being applied to the sphere, 

 produced at one end into a point, and abutting at the other 

 against the third chamber. From this onwards the typical 

 shape is gradually assumed, though in these earlier 

 chambers the alar prolongations are absent. A character 

 of this genus is the presence of the line of pocket-like pro- 

 cesses along the posterior margin of the chambers. It was 

 not clear, until Williamson's paper w-as published, that 

 these ended blindly and did not communicate with the 

 chamber behind. The outer walls of the chambers are 

 traversed by multitudes of pores of extreme minuteness, 

 so that the chainbers of the outer whorl have this addi- 

 tional means of communication with the exterior. There 

 is, besides the structures described, a system of canals, 

 Iving in the thickness of the walls, and communicating 

 with the chambers, but this need not detain us here. 



It results from the structure of such a form as Poly- 

 stomella that in the earliest stage of its existence the whole 

 organism consisted of a single spherical chamber. 



It is to be observed that in shells such as Polystomella 

 the shape and mode of growth of the organism at all 

 stages of its development are preserved in the central parts 

 of the shell. These early formed chambers may be, in 

 some types of growth, exposed to observation, or they may 

 be, as in this genus, built in and hidden by the overlapping 

 of the subsequent additions. They may then, however, be 

 examined by making sections of the shell, or in the proto- 

 plasmic casts of the interior when the shell is dissolved. 



The Foraminifera are found living attached to other 

 objects on the sea bottom from shore pools down to great 

 depths, and from arctic to tropical waters. A small group 

 of them lead a pelagic life suspended in the upper layers 

 of the great oceans, from the surface down, as Dr. 

 Fowler's collections from the Bay of Biscay show, to at 



1 Trais. Mkvoscop. Soc, •. 



NO. 1920, VOL. 74] 



i8<9, p. 159. 



least 500 fathoms, and their empty shells falling to the 

 bottom constitute the large proportion of the grey " Globi- 

 gerina ooze " which in many regions forms the floor of 

 the ocean. 



.An attractive feature of their study is the abundance 

 with which they are represented in geological deposits, 

 right back to the Palaeozoic period, so that in dealing with 

 them w-e have that third dimension, the history of the group 

 in the past, wide open to us in which to project our ideas 

 of the course of their evolution. 



It was from the study of fossil Foraminifera of the early 

 Tertiary period that the recent advances in our knowledge 

 of their life-history received its impulse. 



The later Eocene rocks in many parts of the Viforld 

 abound in discoidal, slightly biconvex Foraminiferous 

 shells, which, from their likeness to coins, have been called 

 Nummulites. The Nummulitic limestones extend across the 

 Old World from the Pyrenees to China, and often attain a 

 thickness of thousands of feet. Visitors to Egypt are 

 familiar with them in the blocks of which the pyramids of 

 Gizeh are built, and the glittering coin-like discs polished 

 by wind and sand and strewn in the desert have attracted 

 notice from remote antiquity. 



The structure of a Nummulite is very similar to that of 

 Polystomella, but the most spacious part of each chamber 

 lies in the median plane of the shell, while the alar pro- 

 longations are very thin and interrupted by supporting 

 pillars of solid shell substance. Hence the median plane 

 is a plane of weakness, and the shell readily splits into 

 planoconve.x halves, the broken surfaces exposing a section 

 in the median plane of all the chambers of which it is 

 built. 



It has long been recognised that while the great niajorit\ 

 of the specimens of Nummulites occurring in a deposit 

 attain a certain moderate size, a few are found scattered 

 through it the diameter of which far exceeds that of the 

 others. On examining median sections of the smaller speci- 

 mens it is usually ' found that the spiral series of chamber^ 

 starts from a large and nearly spherical chamber, readilv 

 visible to the naked eye, and occupying the centre of the 

 shell, while in the large specimens the spiral series is con- 

 tinued to the centre, where in carefully prepared sections 

 it may be seen to take its origin in a spherical chamber 

 of microscopic size. 



Although the two forms were thus found to be associated 

 in the same beds, and to agree with one another closely 

 except in the size to which they grow and in the characters 

 of the central chambers, they were given separate specific 

 names, and attention was called to the puzzling occurrence 

 of these associated " pairs of species," a large and a small 

 one, in various deposits. 



It was especially by the labours of De Hantken and 

 De la Harpe that this' phenomenon was brought to light, 

 the latter pal.-contologist formulating his " Law of the 

 association of species in pairs " as follows : " Nummulites 

 appear in couples ; each couple is formed of two species of 

 the same zoological group, and of unequal size. The large 

 species is without a central chamber, the small always has 

 one." More than sixteen pairs of species of Nummulites 

 and the allied genus Assilina, associated in this manner, 

 have been enumerated. 



In the year 1880 Munier-Chalmas brought before the 

 Zoological Society of France his conclusion that the kinds 

 thus associated were not in fact distinct species but two 

 forms of the same species — that, in fact, the species of 

 Nummulites were dimorphic. He also expressed the 

 opinion that the phenomenon of dimorphism would be found 

 to be of general occurrence among the Foraminifera. 



To this view, which further investigations have shown 

 to be entirelv correct, Munier-Chalmas added a corollary 

 as to the nature of the relation between these two forms, 

 which was wrong. This, however, need not detain us 

 here.- Whether he was set against Munier-Chalmas's 

 views bv the error of part of them, or for whatever reason, 

 De la Harpe failed to recognise, before his untimely death 



1 Usually, because the young of the other type occurs among the smaller 

 specimens. .. . t 1 ■ 



'- Cp. the article bv the author on " Foramimrera in Lankester s 

 "Treatise on Zoology," Part I.. Fa.sc. 2, p. 47 i and "On the Dimorphism 

 of the English Species of Nummulites, &c.,' P.R.S., vol. Ixxvi. B., 

 p. J9S. 



