

BRACHIOPODA. 



BIUCUIOrODA. 



.: .1 



of a minute description ; but its oonroluted intc.itine show* capacity 

 for extracting a quantity of nutriment proportioned to iU superior 

 activity and the extent of iU soft parts. A more complex and obvious 

 respiratory apparatus was therefore indispensable, and it is not sur- 

 prising that the earlier obserren 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 animal* living 

 beneath a pressure of from 60 to 90 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 part i'-li's. and bring within the reach of their prehensile organs 

 the animalcula adapted for their support. The actions of Terebratula 

 and OrbiftJa, 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 divarication or sliding motion of 

 their protecting valves ; and the simplicity of their digestive appa- 

 ratus, the corresponding simplicity of their branchiae, and the 

 diminished proportion of their soft to their hard parts, are in har- 

 mony with such limited powers. The soft parts in both genera are 

 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 superodded to its outward defence, 

 by means of which additional support is afforded to the shell, a 

 stronger defence to the viscera, and a more fixed point of attachment 

 to the brachial cirri. 



" The spiral disposition of the arms is common to the whole of the 

 brachiopodous genera whose organisation has hitherto been examined ; 

 and it is therefore probable that in that remarkable genus Spirifcr, the 

 entire brachia were similarly disposed, and that the internal calcareous 

 spiral appendages were their supports. If, indeed, the brachia of 

 Tcrebralula pnUacta had been so obtained, this species would have 

 presented in a fossil state an internal structure very similar to that of 

 Spirifrr. 



" In considering the affinities of the Brachiopoda to the other orders 

 of Molltuca, I shall compare them, in the first place, with the Lamelli- 

 branchiate Bivalves, to which they present the most obvious relations 

 in the nature and forms of their organs of defence. To these they 

 are in some respects superior. The labial arms are more complex 

 prehensile organs than the corresponding vascular lamina; on either 

 Bide of the mouth of the Lameltibranchiata. The whole muscular 

 system is more complex ; and the opening as well as the closing of 

 the shell being regulated by muscular action, indicates a higher 

 degree of organisation than where the antagonising power results 

 from a property of the cardinal ligament, which is independent of 

 vitality, viz. elasticity. With respect however to the respiratory 

 organs, the modifications which those have presented in Orbicula and 

 Terebratula show the Brachiopods to be still more inferior to the 

 liamrllibranchiala than was to be inferred from the structure of the 

 branchiae in Lin/juht ; and notwithstanding the division of the systemic 

 heart, I consider that there is also an inferiority in the vascular 

 system. Each heart, for example, in the Brachiopoda is as simple as 

 in Atcidia, consisting of a single elongated cavity, and not composed 

 of a distinct auricle and ventricle, as in the ordinary bivalves ; for in 

 these, even when, at in the genus A rca, the ventricles are double, the 

 auricles are also distinctly two in number ; and in the other genera, 

 where the ventricle is single, it is mostly supplied by a double 

 auricle. The two hearts of the flrarhiopoda, which in structure 

 resemble the two auricles in the above bivalves, form therefore a 

 complexity or superiority of organisation more apparent than real. 

 Having been thus led to consider the circulating as well as respiratory 

 systems as constructed on an inferior plan to that which pervades the 

 same important systems in the Lamellibranchiato Bivalves, I infer 

 that the position of the Brachiopoda, in the natural system is inferior 

 to that order of A cephiUa. 



"Among the relations of the Brachiopoda to the Tunicated 

 Afphala, and more especially to the Atcidice, we may first notice an 

 almost similar position of the extended respiratory membranes in 

 relation to the mouth, so that the currents containing the nutrient 

 molecules must first traverse the vascular surface of that membrane 

 before reaching the mouth ; the simple condition, also, to which the 

 l.r.inchiic are reduced in Orbicula and Terebralula indicates their close 

 affinity to the Airulue. But in consequence of the form of the 

 respiratory membrane.* in the liracliiopuda, which is so opposite to 

 that of the iaociform br.uiehiic of the Afiilitr, the digestive system 

 derives no assistance from tliit part as a receptacle for the food, and 

 the siiperadditinn of prehensile organs about the mouth became a 

 neoeswry consequence. The Brachiopods again are stationary, like 

 the Atcidue, and resemble tli /t-i/itniie in the poduiiciilat <! mode of 

 their attaehinant ta foreign 1 



" With the Cirripetlii their relation is one of very remote analogy, 

 their generative, nervous, and respiratory organ* being constructed on 

 a different type, and their brachia manifesting no trace of their arti- 

 culate structure. In all essential joints the Brachiopoda closely 

 correspond with the Acephalous Mollutca, and we consider them as 

 being intermediate to the Lamellibranchiate and Tunicate orders; 

 not however possessing, so far as they ire at present known, a distinc- 

 tive character of sufficient importance to justify their being regarded 

 as a distinct class of Mullusks, but forming a separate group of equal 

 value with the Lamellibranchiata," 



The structure of the shells of the Brachiopoda has lieen atten- 

 tively studied by Dr. Carpenter, and the results of his investigations 

 have been published in his ' Report on the Microscopic Structure of 

 Shells,' made to the British Association. 



The following it De Blaiuvillc's arrangement of the Srachiopoda, 

 slightly modified : 



* 



Shell Symmetrical. 



Terebratula (Bruguierea). Animal depressed, circular or oval, more 

 or leas elongated. Shell delicate, equilateral, Bubtriangular, in|ui- 

 valve, one of the valves larger and more rounded (bombce) than the 

 other, prolonged backwards into a sort of heel, which is sometimes 

 recurved into a kind of hook-like process, and pierced at its extremity 

 by a round hole, but more frequently divided into a fissure more or 

 lea large and of variable form. The opposite valve generally smaller, 

 flatter, and sometimes operculiform. Of that complicated loop or 

 internal support to which the arms are attached we shall presently 

 speak at large. Hinge on the border, condyloid, placed on a straight 

 line, and formed by the two oblique articulating surfaces of the one 

 valve placed between the corresponding projections of the other. A 

 sort of tendinous ligament comes forth from the hole or fissure 

 above described, by which the animal fixes itself to submarine 

 bodies. 



The following is Mr. Owen's description of the peculiar, complrx, 

 and extremely delicate testaceous apparatus, sometimes called ' the 

 carriage-spring' by collectors, attached to the internal surface of the 

 impcrforate valve : 



" The principal part of this internal skeleton, as it may be termed, 

 consists of a slender, flattened, calcareous loop, the extremities of 

 which are attached to the lateral elevated ridges of the hinge ; the 

 crura of the loop diverge, but again approximate to each other as they 

 advance for a greater or less distance towards the opposite margin of 

 the valve; the loop then suddenly turns towards the perforate valve, 

 and is bent back upon itself for a greater or less extent in different 

 species. When the loop is very short and narrow, as in T. ritrm, 

 Brug., there is but a small tendency towards a reflected portion ; but 

 where the loop is of great length and width, as in T. Chilensa, Brod., 

 T. donala, Lam., and T. Sotcerbii, King., the reflected portion is con- 

 siderable. The loop, besides being ,'fixed by it* origins or crura, is 

 commonly attached to two processes going off at right angles from the 

 sides, or formed by a bifurcation of the extremity, of a central process, 

 which is continued forwards to a greater or less extent from the 

 hinge ; but it Li sometimes entirely free, except at its origins, as, for 

 example, in T. riirea. This reflected loop, forming two arches on 

 either side the mesial plane, towards which their concavities are 

 directed, I have figured as it exists in T. Chilcntu and T. Sowtrbii. 

 It is represented of a similarly perfect form in T. dtntata, by M. De 

 Klnin ville in his ' Malacologie ; ' and the same apparatus in T. doriata 

 is very well figured by Chemnitz, by Sowerby, and more recently by 

 O. Fischer de Waldheim. A similar form is also figured in another 

 species of Terebratula by Poll . 



" The arches of the loop are so slender that, notwithstanding their 

 calcareous nature, they possess a flight degree of elasticity and yield 

 a little to pressure ; but for the same reason they readily break if the 

 experiment be not made with due caution. The interspace between 

 the two folds of the calcareous loop is filled up by a strong but exten- 

 sile membrane, which binds them together, and forms a protecting 

 wall to the viscera : the space between the bifurcated process in T. 

 Chilentit is also similarly occupied by a strong aponeurosis. In thin 

 species the muscular stem of each arm is attached to the outer sides 

 of the loop and the intervening membrane. They commence at the 

 pointed processes at the origins of the loop, advance along the lower 

 portion, turn round upon the upper one, and are continued along it till 

 they reach the transverse connecting bar, where they advance again for- 

 wards, and terminate by making a half-spiral twist in front of the 

 mouth. It is these free extremities which form the third arm men- 

 tioned by Cuvier. These anus are ciliate on their outer side fr their 

 entire length, but the cilia ore longer and much finer than the braehial 

 fringes of Lingula; and except at the extreme ends, whic.h have a 

 slight incurvation, they ore uniformly straight. There is thus an 

 important difference between Linguta and those species of Tcrebratnt<i 

 which resemble T. Chilean* in the powers of motion with which the 

 arms are endowed ; since, from their attachment to the calcareous 

 loop, they are fixed, and cannot be unfolded outwards as in Lingula. 

 Owing to this mode of connection, and their ciliated structure, their 

 true nature was much more liable to be mistaken by tin- early observ- 

 ers, though it appears not to have escaped the discrimination of l.in- 

 iwus, who, as Cuvier hus observed, founded his character of the animal 



