[Fig. 102] represents the interior of the Operculina, which is an existing representation of the Nummuline type. Every segment of the animal is enclosed in a shell of its own, but all the segments are connected in the spiral direction by narrow passages in the walls as in the Faujasina.
Although each of the interior whorls has its perforated marginal band, the segments can have no direct access to the water; however, they are indirectly brought into contact with it by means of a system of branching shelly canals, radiating from the central chamber, ending in conspicuous pores in the external surface of the shell. During this course the canals send small tubes into the chambers on each side of them; through these the internal segments of the animal can fill the canals with cords of sarcode, and protrude them into the water, whence they are supplied with food.
The genus Polystomella is distinguished by the high development of the intermediate skeleton and the canal system that maintains it. The Polystomella crispa ([fig. 97], E), a beautiful species common on the British coasts and in other temperate seas, has a lenticular form, the 1⁄16 to the 1⁄12 of an inch in diameter. It consists of a small number of convolutions winding round the shorter axis of the lens, increasing rather rapidly in breadth, and each one almost entirely enclosing its predecessor, so that the shell is exactly alike on both sides, and only the last convolution is to be seen. At the extremities of the axis there is a mass of solid shell-substance, perforated by orifices which are the apertures of a set of straight, parallel canals. In the figure only the last convolution is visible, upon which the convex septal bands are very conspicuous, dividing the surface into well marked segments, upon the exterior edge of each of which there are strong transverse crenulations. The only communication which the chambers have with the exterior, is by means of a variable number of minute orifices near the inner margin of the sagittate partition-plane, close to its junction with the preceding convolution; a very high microscopic power is required to see them, as well as the minute tubercles with which the surface of the shell is crowded, more especially on the septal bands and in the rows of depressions between the segmental divisions.
The sarcode animal itself corresponds exactly with the form and spiral arrangement of the chambers so strongly marked on the exterior of the shell. The segments form a spiral of crescents, smooth on the convex and crenulated on the concave side; and from the latter threads of sarcode proceed, which pass through pores in the inner margins of the partitions, and unite them into one animal.
The Polystomella lives in tropical seas; P. crispa in temperate latitudes, and P. striato-punctata inhabits the polar waters; the genus is found everywhere.
Although variety of form without specific difference is characteristic of the Foraminifera, it sometimes happens that identity of external form is accompanied by an essential difference in internal structure. Of this the Cycloclypeus is an instance; it is a rare species of nummuline, dredged up from rather deep water off the coast of Borneo. The shell is gigantic, some specimens being two and a half inches in diameter; but its mode of growth is the same with that of the most complicated Orbitolite. It consists of three superposed stages of circular discs, each circle of chambers enclosing all those previously formed. However, each segment of the animal being enclosed in its own shelly envelope, a supplemental skeleton, and a radial, vertical and annular system of canals, prove that the two animals belong to essentially different families of Foraminifera. There are many instances, especially in the Rotaline group, of isomorphism accompanied with generic difference; thus no reliance can be placed on variety of external form, unaccompanied by change of internal structure.
An attempt has been made in the preceding pages to describe a few species most characteristic of some of the genera of this multitudinous class; and of those selected a mere sketch of the most prominent features of the animal and its abode is given, that some idea may be formed of the wonderfully complicated structure of beings, which are mostly microscopic specks. Yet the most minute circumstances in the forms of the animals and their shells, with their varieties and affinities, have been determined with an accuracy that does honour to microscopic science.
They are now arranged in a natural system by William B. Carpenter, M.D. F.R.S. assisted by William K. Parker, Esq., and T. Rupert Jones, Esq., and published in the Transactions of the Ray Society in 1862. To this admirable work, the author is highly indebted.
It was known that different types of Foraminifera abound at different depths on the coasts of the ocean; but it was long believed that no living creature could exist in its dark and profound abyss. By deep-sea sounding, it has been ascertained that the basin of the Atlantic Ocean is a profound and vast hollow or trough, extending from pole to pole; in the far south, it is of unknown depth, and the deepest part in the north is supposed to be between the Bermudas and the Great Banks of Newfoundland. But by a regular series of soundings made by the officers of the navies of Great Britain and the United States, for the purpose of laying a telegraphic cable, that great plain or steppe was discovered, now so well known as the telegraphic plateau, which extends between Cape Race in Newfoundland, and Cape Clear in Ireland. From depths of more than 2,000 fathoms on this plateau, the ooze brought up by the sounding machine consisted of 97 per cent. of Globigerinæ. The high state of preservation of these delicate shells was no doubt owing to the perfect tranquillity which prevails at great depths; for the telegraphic plateau and the bed of the deep ocean everywhere is covered by a stratum of water unruffled by the commotion raised by the hurricane which may be raging on the surface. The greater number of the Globigerinæ were dead empty shells; but although in many the animal matter was quite fresh, Professor Bailly of New York could not believe that such delicate creatures could live on that dark sea bed, under the pressure of a column of water more than 2,000 fathoms high, a weight equal to rather more than that of 340 atmospheres or 5,100 lbs. on every square inch of sea-bed; wherefore he concluded that the tropical ocean and the Gulf Stream, which absolutely swarm with animal life, must have been the birth-place and home of these minute creatures, and that this mighty ‘ocean river,’ which divides at the Great Banks of Newfoundland, and spreads its warm waters like a fan over the north Atlantic, deposits their remains over its bed, which has thus been their grave-yard for unknown periods, and which, in the lapse of geological time, may be raised above the waves as dry land.
Professor Ehrenberg on the contrary concluded that residentiary life exists at the bottom of the ocean, both from the freshness of the animal matter found in the shells, and from the number of unknown forms which are discovered from time to time at various and often great depths along the coasts. This opinion has been confirmed beyond a doubt on several occasions, especially by Dr. Wallich, who accompanied an expedition sent under the command of Sir Leopold M‘Clintock, to sound the North Atlantic for laying a telegraphic line.