Page images
[graphic][subsumed][ocr errors]
[ocr errors][merged small][merged small][merged small]

I. A Monograph of Recent Brachiopoda.—Part I.
By Thomas Davidson, LL.B., F.R.S., F.L.S., F.G.S., fyc.

Head 5th November, 1885.

(Plates I.-XIII.)

Introductory Remarks.

DURING the last hundred years the recent Brachiopoda have attracted considerable attention, and a large number of valuable memoirs and papers have been published upon them. Their shells, shell-structure, anatomy, embryology, and affinities have alike been carefully investigated. Observations on the living animals of several genera have also been recorded. The sea-bottoms have been dredged for Brachiopoda in many latitudes and over a wide geographical area, and their habitats and ranges of depth accurately ascertained to a very considerable extent. Four or five incomplete monographs, in which the shells only of a large number of species have been well illustrated and briefly described, have appeared during the present century; but no satisfactory general monograph treating of the shell and animal conjointly has yet been published. This omission I have now endeavoured to supply.

In 1843, Kiister, in his new edition of Chemnitz's 'Conchy lien-Cabinet,' described some twenty-six or thirty species, of which several are now known to be synonyms. These he figured in six quarto plates.

In 1846, G. B. Sowerby, in his 'Thesaurus Conchyliorum,' described and beautifully illustrated forty-seven species, of which number several are synonyms.

In 1859-62, Lovell Reeve, in his 'Conchyliorum Iconica,' described the shells of seventyfive species, of which some were synonyms, accompanied with a series of beautiful illustrations, drawn by G. B. Sowerby.

In 1873, in the 'Proceedings of the Academy of Natural Sciences of Philadelphia,' Mr. W. H. Dall published a catalogue of all the recent species of Brachiopoda known to


him up to that date. In this catalogue without figures, about one hundred species are enumerated, some of which are synonymous.

During the last thirty-five or more years, I have devoted much time to the study of the recent forms, in conjunction with that of the fossil species, and have lost no opportunity of making myself acquainted with all that has been done and written upon the subject, as well as in assembling all available material, so as to enable me to bring together in a single monograph the chief results of many independent researches published in a number of scattered papers and works often difficult of access. The literature of the subject is indeed voluminous, as may be realized by a glance at the 'Bibliography of the Brachiopoda,' compiled by Mr. W. H. Dalton and myself, and published in vol. vi. of my * British Fossil Brachiopoda' (Palaeont. Soc, 1886).

I have also, I believe, had advantages which few have possessed in being able to follow out the observations made with respect to the animal and its anatomy, and in having been able to draw a very large number of figures from the types of the best-preserved examples of almost all the known forms, as well as of a large series of individuals of the same species at different stages of development. The study of the adult condition of a species gives insufficient data, and it is requisite to follow out the modifications it has to go through during the different stages of its existence, and to note these differences.

The study of the embryo has also shown that the animal assumes a series of welldefined stages in its development, a fact that was but little known prior to the publication in 1861 of Prof. Lacaze-Duthiers's admirable memoir on Thecidium mediterraneum. These observations were subsequently followed by the excellent researches of Fritz Miiller, Kowalevsky, E. Morse, H. Friele, M°Crady, Dall, Van Bemmelen, A. E. Shipley, M. A. Schulgin, and one or two more. The results obtained by these authors will be referred to in the sequel. It is very desirable that these important investigations should be continued, as much still remains to be discovered, described, and illustrated.

The shell-structure of the recent Brachiopoda has been admirably worked out by a number of accurate observers, such as Dr. W. B. Carpenter, W. King, Van Bemmelen, Hancock, and many others, and has led to very important results. To Herman Friele, E. Deslongchamps, and one or two others we are indebted for much accurate and important knowledge with respect to the development of the loop, of which but little was known previous to 1852.

The anatomy of the animal has also been admirably investigated and worked out, and it is sufficient to mention the names of Cuvier, Owen, Huxley, Hancock, Vogt, Gratiolet, Lacaze-Duthiers, King, Brooks, Dall, Morse, E. Deslongchamps, Van Bemmelen, Woodward, Shipley, Schulgin*, and others, to show how important and varied have been the additions to our knowledge with respect to this very necessary branch of investigation. In drawing up the description of each species, I have considered it desirable, whenever possible, to reproduce the words and illustrations of the authors, and thus give them all credit for their careful, painstaking researches.

* To these names Dr. Davidson would doubtless have added that of H. G. Beyer, who contributed an important paper on the shell-structure and anatomy of Lingula (Glottidia) jryramidata, Stimpson, to the Studies from the Biological Laboratory of the Johns Hopkins University, Baltimore, vol. iii. no. 5, March 1SS6.—[A. C]

The perplexing question of the affinities of the Brachiopoda has given rise to much discussion, and great difference of opinion, especially with regard to their relationship to the group of worms. Now, although I do not admit the Brachiopoda to be worms, they may, as well as the Mollusca and some other groups of invertebrates, have originally diverged from an ancestral vermiform stem, such as the remarkable worm-like mollusk Neomenia would denote. In a recent paper on the development of Argiope or Cistella, Mr. A. E.Shipley observes, and, I think, with justice, that the Brachiopoda and Polyzoa are not so closely united as to form a natural phylum; and he adds, " I should propose to follow Gegenbaur in making a primary class of the Brachiopoda, and though in their development and adult structure they are widely separated from both Vermes and Mollusca, of the two classes I would place them nearer to the former class than to the latter " *. Prof. Huxley f says :—" All known Polyzoa are compound animals, that is to say, the product of every ovum gives rise, by gemmation, to great assemblages of partially independent organisms, or zooids. The Brachiopoda, on the contrary, are all simple, the product of each ovum not giving rise to others by gemmation. All the Brachiopoda possess a bivalve shell—a shell composed of two, more or less horny, or calcified, pieces, which are capable of a certain range of motion on one another, and are very commonly articulated together by teeth and sockets." The shell, the pallial lobes, the intestine, the nerves, and the atrial system, afford characters amply sufficient to define the class.

In this view of Prof. Huxley I entirely concur.

As many species of Brachiopoda live at considerable depths, it is not surprising that so small a number should have been known to early conchologists, and that for many years they should have been such great rarities in conchological collections. The numerous well-conducted dredging expeditions have, however, brought to light a large number of forms that were not previously known, and we may constantly expect to add to the number of species as dredging operations extend to regions not yet explored. It has been ascertained beyond doubt that Brachiopoda are much localized, and that where they occur they are generally abundant. It has also been found that the range in depth of one and the same species is often very variable, that abyssal forms have generally a very thin shell, and that species living at a great depth have a much greater geographical range, and are not nearly so localized as those species that live in shallow waters.

The study of the species brought home by the 'Challenger' Expedition, which I was privileged to examine and describe, has revealed much valuable information with respect to the bathymetrical and geographical distribution of many species. The greatest depth at which a recent species of the class has been found alive was 2900 fathoms. A number of forms inhabit and prefer rocky and stony parts of the bottom, or are attached to corals, and are therefore more difficult to obtain.

It is necessary briefly to refer to the difficult question of classification, upon which many different opinion^ have been entertained. In company with a larger number of

* "On the Structure and Development of Argiope." Mittheilungcn aus der zool. Station zu Neapel, Band iv. Heft 4, p. 516 (1883).

t An Introduction to the Classification of Animals, ]>. 27 (1869).


malacologists and palaeontologists, I have considered the interior skeleton that supports the labial appendages as a classificatory character that could be advantageously made use of, and have consequently grouped the recent species into the two great divisions Arihropomata, Owen ( = Clistenterata, King), and Lyopomata, Owen (= Tretenterata, King), and into six families, as follows:

[merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][ocr errors][ocr errors][ocr errors]

6th Family LINGULID.E f XXL Genus Lin^ta' Bruguiiro

I XXII. Subgenus Glotlidia, Dall .

In 1884, M. E. E. Deslongchamps proposed a new scheme of classification for the Terebratulidae, in which he objected to any arrangement based on either the exterior shape of the shell or of the supports of the labial appendage.

His first group includes the different forms in which the calcified brachial apparatus or loop does not undergo any important modifications from its first origin up to the adult condition. To the characters drawn from the brachial apparatus or loop is added that of the presence of spicula, more or less complicated, which occupy in the mantle all the parts connected with the organs of circulation (arteries, veins or veiny sinuses, &c), the labial appendages and cirri which accompany them. In this group he places the recent genera Liothyris, Terebratulina, Megerlia, Kraussina, and Platydia.

« PreviousContinue »