"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 Spinier, the entire brachia were similarly disposed, and that the internal calcareous spiral appendages were their supporta. If, indeed, the brachia of Terebratula prittacea had been so obtained, this species would have presented in a fossil state an internal structure very similar to that of Spirifer.
" In considering the affinities of the Brachiopoda to the other orders of .11o1lusea, 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 lamiwe on either side of the mouth of the Lamellibranchiata. The whole muscular system is more complex ; and the opening as well as the closing of the shell being regulated by muscular action, indicates n 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 these have presented in Otticula and Tercbratula show the Brachiopods to be still more inferior to the Larnellibranchiata than was to be inferred from the structure of the bronchia, in Linyula ; 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 Braehiopoda is as simple as in Aseidia, 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, as in the genusA 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 Brachiopoda, 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 some important systems in the Lamollibranchiato Bivalves, I infer that the position of the Brachiopeda in the natural system is inferior to that order of A cephata.
"Among the relations of the Brachiopoda to the Tunicated Acephala, and more especially to the Aacidia', 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 vaiettler surface of that membrane before reaching the mouth ; the simple condition, also, to which the branchim are reduced in Orb le ula and Teed's/tido indicates their close affinity to the Aseidite. But in consequence of the form of the
respiratory membranes in the Brachiopoda, which is so opposite to that of the s.seciform branehime of the A seidier, the digestive system derives no assistance from thet part as a receptacle for the food, and the auperaddition of prehensile organs about the mouth became a necessary consequence. The Brachtopode again are stationary, like the Aseedim, and resemble the Bottcnew in the poduuculatad mode of their attachment to foreign 'radials.
"With the Cirripeda their relation is one of very remote analogy, their generative, nervous, and respiratory organs being constructed on a different type, and their brachia tuanifesting no trace of their arti culate structure. In all essential points the Brachiopoda closely correspond with the Acephalous Jtollasea, and we consider them as being intermediate to the Latnellibranehiato and Tunicato orders ; not however poisseming„ so far as they are at present known, a distinc tive character of sufficient importance to justify their being regarded as a distinct class of Mollusks', but forming a separate group of equal value with the Lamdlibranehiata." The structure of the shells of the Brachiopoda has been 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 Is De Blainvillo's arrangement of the Brachiopods, slightly modified • Shell Symmetrical Terebratula (Bruguierea). Animal depressed, circular or oval, more or less elongated. Shell delicate, equilateral, subtriangular, inequpvalve, one of the valves larger and more rounded (bomb6c) 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 fiasuro more or less 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, coudyloid, 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.