Brachiopod

“ Like the sedentary worms, and most of the mollusks the brachiopod starts life as a minute, free-swimming, wormlike larva, top-shaped and extremely active. During this period the mortality of the tiny unprotected creatures is very great, but once the mud-dwelling phase of existence has started, the race is secure from most enemies. ”

“ The digestive system of a brachiopod includes an œsophagus that leads into a simply formed stomach round which is a large digestive gland. The heart has only one cavity, but the animal is provided with two smaller and separate organs that assist in the propulsion of the blood, which circulates through numerous blood spaces in the bristly mantle.
About two thousand fossil species of brachiopods are known, extending over a vast range of time; and the living species, numbering less than a hundred, are found from shallow water to the greatest habitable depths. ”

“ In the whole world there are about 110 extant species compared to nearly 500 which Dr. G. Arthur Cooper, Smithsonian Institution curator of invertebrate paleontology, and his associates have obtained from one small area of the Glass Mountain limestones.
The existing brachiopod might be mistaken for a small clam. Zoologically, it is an intermediate form between mollusks and annelid worms, and somewhat closer to the latter than the former. Its way of life actually is nearer to that of an oyster than to that of most worms. ”

“ Their proportional numbers can by no means be explained by supposing them to have inhabited seas of great depth, for the contrast between the palæozoic and the present state of things has not been essentially altered by the late discoveries made in our deep-sea dredgings. We find the living brachiopoda so rare as to form about one forty-fourth of the whole bivalve fauna, whereas in the Lower Silurian rocks of which we are now about to treat, and where the brachiopoda reach their maximum, they are represented by more than twice as many species as the Lamellibranchiate bivalves. ”

“ Throughout a limestone series forty feet thick thin waterlime layers occur and it is in these alone that the films of eurypterid
exoskeletons are found. In the pure limestones a brachiopod fauna occurs, but no eurypterids are present; while in the separating waterlime eurypterids and ceratiocarids, but no brachiopods are found. Foerste has made the following statements in regard to the occurrence: "At the McReynold or Interurban quarry, in the southwestern corner of Kokomo, there is a much thicker exposure of the upper or brachiopod horizon. No merostomata have been found here. ”

“ Passing from the Azoic rocks, in the northern part of the State of New York, through the Potsdam region, to Trenton Falls, southwardly to the Helderberg Mountains near Albany, and eastwardly to Niagara, the immense number of fossil shells, particularly Brachiopods (Fig. 145) , attracts the attention of the traveler. The Brachiopods of the present seas are few and far between, whereas the sea of that most ancient period was characterized by shells of this order; the remains of other Mollusca are found, but much less abundantly as compared with those of Brachiopods. ”

“ Of course, in their younger days, the wandering larvæ may be swallowed up in numbers by other animals, but when the young brachiopod is once fixed, the shell, the mantle-edge with its bristles, and the ciliary apparatus protect it so well, that not even parasites can attack it. ”

“ Nor is it impossible that some modern bivalves of the Brachiopod group may be scarcely modified descendants even of Palæozoic species.
Perhaps some of the most remarkable facts in connection with the permanence of species and varietal forms are those furnished by that magnificent flora which burst in all its majesty on the American continent in the Cretaceous period, and still survives among us even in some of its specific types, I say survives; for we have but a remnant of its forms living, and comparatively little that is new has probably been added since. ”

“ The fossils, save in the limestone bands, are not easy to find, but in places they are fairly abundant. Brachiopods are far more numerous than the Mollusca properly so-called. Of these, the genus Orthis was most abundant at about the close of this period. Certain beds of this age have received the name of Lingula Flags, owing this prevalence in them of the curious Brachiopod Lingula so like the species now living in some of the warm seas of the tropics. ”

“ The body cavity of a Brachiopod is traversed by several pairs of muscles, which are very constant in position, and whose contraction serves to open and close the shell, to move the animal upon its stalk, and to govern the movements of the arms. ”

“ The organic remains are not, as a rule, in a good state of preservation; however, out of 25 genera and species of Mollusca belonging to the Sand bed, I have recognized as many as 13 in the Marl. The only Brachiopod of the two upper formations that I have likewise found in the Marl is Terebratula sinuosa, which is not uncommon, and is the only representative of the group I have seen from this bed. ”

“ The soft parts are composed of muscles, the mantle (which secretes the valves) , digestive, respiration, reproductive, and excretory organs, and the tentacle-bearing lophophore.
In adult life the brachiopod is attached to the sea bottom by means of a fleshy stalk called the pedicle (fig. 14) , and this is usually extruded through a hole (the pedicle foramen) which is located in the ventral or pedicle valve. The upturned area which is usually present on the pedicle valve is called the beak. ”

“ The hinge characters of the brachiopods, their internal arm supports, their spires and loops, the distribution of the ramifying blood channels in the mantle, the surface markings of every rank and grade down to the smallest which can be observed only with the lens, and the microscopic structure of the shell itself, are other examples of the faithfulness with which details, however insignificant in point of magnitude, have been guarded, protected, preserved. ”

“ The valves are, consequently, essentially symmetrical, which is not the case with the Lamellibranchiata,—so much so, that certain Brachiopod shells were named Lampades, or lamp shells, by some early naturalists; but while such may bear a kind of resemblance to an antique Etruscan lamp, by far the larger number in no way resemble one. The shell is likewise most beautiful in its endless shapes and variations. In some species it is thin, semi-transparent and glassy, in others massive. ”

“ Such is the general arrangement of the shell muscles in the division composing the articulated Brachiopoda, making allowance for certain unimportant modifications observable in the animals composing the different families and genera thereof. Owing to the strong and tight interlocking of the valves by the means of curved teeth and sockets, many species of Brachiopoda could open their valves but slightly. In some species, such as Thecidea, the animal could raise its dorsal valve at right angles to the plane of the ventral one (fig. 4) . ”

“ While a form of Brachiopod may persist nearly unchanged from the lowest geological horizons to the present day, other forms of life may pass through rapid changes and become extinct. A group may go through slow and even changes like the ammonites of the Jura and finally culminate in rapid and extraordinary modifications in form. ”

“ In Discinisca and Lingula, however, the sub-oesophageal ganglion is not drawn out, but lies medianly; it gives off two posteriorly directed nerves to the stalk, which in Lingula unite and form a substantial nerve. Sense organs are unknown in the adult. The larval forms are provided with eye-spots, but no very specialized sense organs are found in the adult.
The histology of Brachiopods presents some peculiar and many primitive features. As a rule the cells are minute, and this has especially stood in the way of embryological research. ”

“ The Victorian mudstone at Heathcote may be of Ordovician age or even older; it has afforded a limited fauna of brachiopods and trilobites, amongst the former being various species of Orthis, Chonetes, and Siphonotreta. The latter genus is represented in both the Lower and Upper Ordovician rocks of slaty character in Victoria (Fig. 85 B) .
Silurian Brachiopods.—
The Silurian system in Australasia as in Europe, N. America and elsewhere, is very rich in brachiopod life. It is impossible to enumerate even all the genera in a limited work like the present, the most typical only being mentioned. ”

“ While few of the recent Brachiopods reach a large size, some of the extinct species measure several inches in breadth, but the great Productus giganteus attained the width of even a foot.
The bright colours of the shells of the living animals are not generally preserved amongst the fossil species from the older rocks; yet in a Carboniferous Terebratula we can even now detect the purple bands in some specimens, and a Cretaceous Rhynchonella similarly exhibits its original colour. ”

“ It is quite possible that, when the lagoon became too saline, they withdrew into the brackish water zone of estuaries or deltas. This inference agrees well with the meagerness of the marine brachiopod and mollusk fauna with which they are associated, the brackish water being still today the least inhabited zone of the hydrosphere on account of its frequent changes in salinity; and it is also in full accord with the occurrence of the eurypterids in the Devonic rocks. There is no longer any doubt that the Stylonurus of the Catskill beds inhabited an estuary with brackish water conditions ”

“ The Molluscoidea, to which phylum belong the Bryozoa and Brachiopoda, would be but imperfectly understood from a morphological standpoint but for the vast number of fossil forms. The Brachiopoda have been estimated to have less than 150 living species, while probably more than 6,000 fossil species have been described. Of the 31 families only 7 have living representatives. We are dependent, therefore, largely on the fossil forms for our knowledge of the morphology of this class. ”

“ When once settled down it has little to fear from the attacks of other animals. The size of its shell relative to its body would deter most animals from regarding it as a desirable article of food, and as far as is known at present the Brachiopoda suffer but little from internal parasites, the only case I know being a minute parasitic Copepod belonging to a new and as yet unnamed genus which I found within the mantle cavity of Cistella (Argiope) neapolitana in Naples. ”

“ The organs of generation also lie in the mantle, and are shown in the drawing as two lumps in the raised back valve. Thus, small is the body in proportion to the vigorously developed arms, from which the name of the whole class, brachiopods, is derived. But are they really arms? They are hardly movable. If we stick a needle into one of them, it does not stir; and there are species in which they are completely calcified. Only the fringes move and respond immediately to excitations. ”

“ The wide distribution and vast abundance of the Brachiopoda throughout the whole series of geological formations make this group of especial importance to the student of the past history of the earth; and the zoologist must always regard the fossil forms with peculiar interest, because they not only largely outnumber the living representatives, but comprise numerous extinct genera, and even families, exhibiting types of structure and characters entirely absent in the modern members of the group. ”

“ Although the Brachiopoda, or "Lamp-shells," are not true mollusca, they are not very far removed from them, and are so often to be found in cabinets that it will not do to pass them over, especially since in past times they were very abundant, an enormous number occurring in the fossil state. Only eight genera are now living. ”

“ Fig. 4. A species of Terebratula (T. diphya, of Lamarck) . The shells of this genus belong to that division of mollusks termed Brachiopoda (arm-feet) , from their having internally two spiral fleshy arms developed from the sides of the alimentary orifice. These organs are supported by shelly processes, curiously modified in different genera, which often occur in a fossil state. Although the fossil Terebratulæ are very numerous, the recent species are but few, and are inhabitants of the seas off Australia. They form two natural groups ”

“ In favour of the theory of the simultaneous life of the host and the parasite, the theory of the growth of the Algæ in recent times must be advanced and the presence of sea-water and of animal tissue of a low vitality assumed. In addition there is the fact that a portion of a Brachiopod included in a sandy matrix within the coral, and not continuous with coral-structures, contains the tubes. Moreover the crystalline mass of the interstices of the coral, although it holds mechanically abundance of spores, does not present tubes or any evidence of growth. ”

“ With the exception of the genus Crania, it is usual for Brachiopods to bear round the edge of their mantle rows or bundles of chitinous setae or bristles (Figs. 315 and 319) . The length and arrangement of these structures vary in the different species; they are secreted from little pits in the edge of the mantle. It seems probable that they serve to some extent as organs of defence, especially in the larva, where they make their appearance at an early stage; possibly they also serve as a filter, and prevent the entrance of foreign bodies into the shell. ”