BRACHIO'PODA, or Brachiopodons Mollusca, Cuvier's fifth class of Mollusks, the Palliobranchians (Palliobranchiata of De Blainville), being the first order of De Blainville's third class of Mollusks (Aeephalophora).
This class, though comparatively low in the scale of creation, is interesting to the physiologist, and of considerable Table to the geologist, who finds in the fossil forms no small portion of those natural medals which indicate the history of the stratification of our globe. Comparatively few of the species exist in the seas of the present day, but in former periods of the earth's surface they occupied the position now taken by the Lamellibmnchiate Nausea.
Cuvier, in his anatomy of Lingula anatina, in the Annales du Musdum,' first made known that organisation by which the mantle, in addition to its office of secreting tho shelly defence of these bivalves, is made subservient to the circulating system. Instead of the branchim of the ordinary bivalves he found in the situation usually occupied by them two fringed and spirally-disposed arms, and that the branchim presented themselves on the internal surface of both lobes of the mantle in oblique parallel lines. He further found that these) lobes were traversed by vessels of considerable size, which returned the blood from the organs of respiration, and that these branchial veins terminated in two symmetrical epitomic hearts. Here was a new type of circulation, and to the mollusks which presented these interesting and important modifications he gave the name at the head of our article, significative of the fringed arms which in this class took the place of the foot or organ of progression in the cockle, &c.
Lamanon and Walsh had previously taken the analogous parts of Terebratula for branchim, and Pallas, who is not quoted by Cuvier, describes the arms of Te rcbrat ula minuteness and accuracy,but consider)) them as branchim, and compares them to those of a fish.
Do Blainville, in the Dictionnaire des Sciences Naturclles,' gives an account of the organisation of Terebratula. But both Cuvier and De Blainville were led into error in their attempts to trace out some parts of the organisation of Terebratula; and it was reserved for Mr. Owen, in his acute, accurate, and interesting paper `On tho Anatomy of the Brachiopoda of Cuvier, and more especially of tho Genera Terebratula and Orbicula,' published in the ` Transactions of the Zoological Society of London' (vol. i. p. 145), and derived from the dissection of specimens brought to this country by Mr. Cuming and Captain James Ross, R.N., fully to investigate the subject so as to leave little or nothing to be desired upon the subject of the anatomy of Lingula and of the two genera last named. Our limits will not permit us to follow the learned author through his memoir, the whole of which, together with the beautiful illustrations that accompany it, is worthy of the most attentive perusal by the physiologist and zoologist. The following general remarks from Professor Owen's paper illustrate his views. It should be premised that the Brachiopoda are cryptandrous "On comparing together," says Mr. Owen, "the three genera of Brachiopoda above described, we find that although Orbicula, in the muscular structure of its arms and the proportion of the shell occu pied by its viscera, is intermediate to Lingula and Terebratula, yet that in the structure of its respiratory organs, its simple alimentary canal, and its mode of attachment to foreign bodies, it has a greater affinity to the latter genus. The modifications that can be traced in
the organisation of these genera have an evident reference to the different situations which they occupy in the watery element. Lingula, living more commonly near the surface, and sometimes where it would be left exposed by the retreating tide, were it not buried in the sand of the shore, must meet with a greater variety and abundance of animal nutriment than eau bo found in those abysses in which Terebratula is destined to reside. Hence its powers of prehen sion are greater, and Cuvier suspects it may enjoy a species of loco motion from the superior length of its pedicle. The organisation of its mouth and stomach indicates however that it is confined to food of a minute description ; but its convoluted intestine shows a capacity for extracting • quantity of nutriment proportioned to its superior activity and the extent of its soft parts A more complex and obvious respiratory apparatus was therefore indispensable, and it is not sur prising that the earlier observers failed to detect a corresponding organisation In genera destined to a more limited sphere of action. The respiration indeed as well as the nutrition of animals living beneath a pressure of from 60 to 00 fathoms of sea water, are subjects of peculiar interest, and prepare the mind to contemplate with less surprise the wonderful complexity exhibited in the minutest parts of these diminutive creatures. In the stillness pervading these abysses they can only maintain existence by exciting a perpetual current around them in order to dissipate the water already loaded with their effete particles, and bring within the reach of their prehensile organs the animalcule adapted for their support. The actions of Terebratula and Orbicsda, from the firm attachment of their shells to foreign sub stances, are thus confined to the movements of their brachial and branchial filaments, and to a slight divarioation or sliding motion of their protecting valves ; and the simplicity of their digestive appa ratus, the corresponding simplicity of their branchim, and the diminished proportion of their soft to their hard parts, arc in har mony with such limited powers. The soft parts in both genera arc , however remarkable for the strong and unyielding manner in which they are connected together. The muscular parts are in great pro portion and of singular complexity, as compared with ordinary bivalves ; and the tendinous and aponeurotic parts are remarkable for the similarity of their texture and appearance to those of the highest classes. By means of all this strength they are enabled to perform the requisite motions of the valves at the depths in which they are met with. Terebratula, which is more remarkable for its habitat, has an internal skeleton superadded to its outward defence, by means of which additional support is afforded to the she]], a stronger defence to the viscera, and a more fixed point of attachment to the brachial cirri.