According to Agassiz, the transition between such types as Pedicellina to Membranipora and other incrusting Polyzoa is readily explained from the embryology of Thecidium, and, in fact, all incrusting Polyzoa are only communities of Brachiopods, the valves of which are continuous and soldered together, the flat valve forming a united floor, while the convex valve does not cover the ventral one, but leaves an opening more or less ornamented for the extension of the lophore. Both Gratiolet and Hancock have expressed the opinion that the Tunicata are in no way related to the Brachiopoda, and that we cannot place these last and'the Polyzoa along-with the Tunicata in the same division. Gratiolet and some others have considered the Brachiopoda to be allied to the Crustacea, while even the asteridian affinities of the class have been hinted at by King.* t

No doubt can be entertained, after perusing the admirable memoirs by Morse and Kowalevsky on the embryology of Terebratula, Terebratulina, Argiope, and Thecidium, that the genera composing the class and Amphetrite possess many important features in common, but almost any Invertebrate groups might be annelidilized by overrating certain points in their affinities. Mr Dall thinks that the general conclusion with reference to the affinities of the Brachiopoda will be something like this. There is much reason for supposing that all the Molluscs and Molluscoids came from the stock out of which the Worms have developed. Indeed, as Huxley has said, they are only isomerous Worms with many special modifications. It is natural, therefore, that the oldest and lowest forms should retain many of the characteristics of the oldest and most simple Worms, especially those which have been modified by a tubular habit. But, on the whole, the modifications are so important that we may continue to consider (if in the specializing tendency of present study we can retain any general divisions of Invertebrates) that the Molluscoids and Molluscs do form two groups somewhat aside from others, and somewhat more nearly related to each other than to the divisions external to them. Therefore, although it may turn out that the Brachiopoda constitute a class close to the Annelids, it cannot be denied that they possess many molluscan characters that cannot be overlooked, and are, under any circumstances, entitled by their importance and numerous distinctive features to constitute a well-defined and separate class.

The name Brachiopod (βpαχiωv, an arm, τroυs, πoδ<k, a foot) was proposed for the class by Cuvier in 1805, and by Dumeril in 1809, and has since been very extensively adopted. Blainville in 1824 proposed as a substitute for the Cuvierian name that of Palliobran-chiata (pallium, a mantle ; branchice, gills), on account of the respiratory system being combined with.the mantle on which the vascular ramifications are distributed. Prof. King has always adopted the latter name, and perhaps rightly objects to Cuvier’s on the ground that it is a misnomer, for the two variously curved and cirratcd brachial or labial appendages, improperly designated as arms or feet, were subsequently found not to subserve the function of locomotive organs.

Before describing the various parts of the animal and its shell, it may be as well to mention that it had been many times suggested by Owen, Bronn, Huxley, Gill, and others, that the class could be advantageously divided into two primary groups. Thus, for the first division, including Lingula, Discina, &lc., the names Lyopomata (Owen, 1858),

Pieuropygia (Bronn, 1862), Inarticulata (Huxley, 1869), Lyopomata (Gill, 1871), have been made use of; while for the second division, comprising Terebratula, llhyn-chonella, &c., the names Athropomata (Owen, 1858), Apygia (Bronn, 1862), Articulata (Huxley, 1869), Arthro-pomata (Gill, 1871) have been proposed. Prof. King, considering these names to be objectionable, and in some cases inadmissible on certain grounds, in 1873 substituted the name Tretenterata for the first group, the intestine being provided with an anal aperture, while the second group, to which he gives the name of Clistenterata, would embrace animals that are destitute of that organ; but it must also be remembered that the presence or absence of an anal aperture has been ascertained in only three or four recent genera, and that we are compelled, in a measure, to take for granted that what we find to be the case in Lingula and Terebratula is also so in the many extinct families and genera of which the animal cannot be examined.

With the character above enounced, we generally find structural modifications of the hinge and other differences in the animal, and especially so in what relates to the muscular system. In the opinion of Prof. King the absence of an anal vent in the Clistenterata makes them inferior to the aniferous Tretenterates.

The animal of the Brachiopod is in all cases protected by a shell composed of two distinct valves; these valves are always, except in cases of malformation, equal-sided, but not equivalved. The valves are, consequently, essentially symmetrical, which is not the case with the Lamellibranchiata or Conchifera,—so much so, that certain Brachiopod shells received the name 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. Generally the shell is from a quarter of an inch to about four inches in size, but in certain species it attains nearly a foot in breadth by something less in length, as is the case with Productus giganteus. The valves are also in some species very unequal in their respective thickness, as may be seen in Productus Llangol-lensis, Daυidsonia Verneuιlii, φc., and while the space allotted to the animal is very great in many species, as in Terebratula spliceroidalis, it is very small in others belonging to Strophomena, Leptcena, Chonetes, &c. The ventral valve is usually the thickest, and in some forms is six or seven times as great as the opposite one. The outer surface of many of the species presents likewise the most exquisite sculpture, heightened by brilliant shades, or spots of green, red, yellow, and bluish black. Traces of the original colour have also been preserved in some of the fossil forms; radiating bands of a reddish tint have been often seen in well-preserved examples of Terebratula hastata, T. saculus, T. communis, T. biplicata, and of several others. Some specimens of T. carnea are of a beautiful pale pink colour when first removed from their matrix, and E. Deslongchamps has described the tint of several Jurassic species.

The valves have been distinguished by various names, but those of dorsal and ventral are in most general use. The ventral valve is usually the largest, and in many genera, such as Terebratula and Phynchonella, has a prominent beak, with a circular or otherwise shaped perforation or foramen at or near its extremity, partly completed by one or two plates, termed a deltidiuιn. Through the foramen passes a bundle of muscular fibres, termed a peduncle, by which the animal -is in many species attached to submarine objects during at least a portion of its existence. Other forms show no indication of ever having been attached, while some that had been moored by means of a peduncle during the early portion of their existence have become detached at a more advanced stage of life, the opening becoming gradually cicatrized, as is so often seen in Strophomena rhomboidalis, Orthesina anomala, <fcc. Lastly, some species have adhered to submarine objects by a larger or smaller portion of their ventral valve, as is the case with many forms of Crania, Thecidium, Davidsonia, <fcc. Some Cranias are always attached by the whole surface of their lower or ventral valve, which models itself and fills up all the projections or depressions existing either on the rock, shell, or coral to which it adhered. These irregularities are likewise, at times, reproduced on the upper or dorsal valve. Some species of Strophalosia and Productus seem also to have been moored during life to the sandy or muddy bottoms on which they lived, by the means of tubular spines of greater or lesser length. The interior of the shell varies very much according to families and genera. On the inner surface of both valves several well-defined muscular vascular and ovarian impressions are observable; they form either indentations of greater or lesser size and depth, or occur as variously shaped projections. In the Trimerellidoe, for example, some of the muscles are attached to a massive or vaulted platform situated in the medio-longitudinal region of the posterior half or umbonal portion of both valves. In addition to these, there exists in the interior of the dorsal valve of some genera a variously modified, thin, calcified, ribbon-shaped lamina or skeleton for the support of the labial or brachial appendages; and so varied, yet constant in shape to certain species is this laminal apophysis, that it has served as one of the chief characters in the creation of both recent and extinct genera. The apophysis is more or less developed in some genera than in others. In certain forms, as in Terebratula, and Terebratulina, it is short and simple, and attached to a small divided hingeplate, the two riband-shaped lamina being bent upwards in the middle (fig. 15). The cardinal process is prominent, and on each side of the hinge-plate are situated the dental sockets; the loop in Terebratulina becomes annular in the adult by the union of the oral processes or crura (fig. 16). In Waldheimia it is elongated and reflected; the hingeplate large, with four depressions, under which originates a mesial septum, which extends more or less into the interior of the shell (figs. 13 and 14). In Terebratella the loop is attached to the hinge-plate and to the septum (fig. 17). In Megerlia it is three times attached, first to the hingeplate, and then to the septum by processes from the diverging and reflected positions of the loop. In Magas the apophysary system is composed of an elevated longitudinal septum reaching from one valve to the other, to which are affixed two pairs of calcareous lamellae, the lower ones riband-shaped; attached first to the hinge-plate, they afterwards proceed by a gentle curve near to the anterior portion of the septum, to the sides of which they are affixed; the second pair originate on both sides of the upper edge of the septum, extending in the form of two triangular anchorshaped lamellae (fig. 18). In Bouchardia the septum only is furnished with two short anchor-shaped lamellae. Many more modifications are observable in different groups of which the great family Terebratulïdae is composed, and which will be found fully described in Davidson’s and other authors’ works on the Brachiopoda. In Thecidium, (figs. 3, 4) the interior of the dorsal valve is variously furrowed to receive a testaceous ridge folded in two or more lobes. In the family Spiriferidae there are two conical spires directed outwards, and nearly filling the cavity of the shell (fig. 5); while in Atrypa the broad spirally coiled lamellae are vertical, and directed towards the centre of the valve. In the Bhynchonellidae there are two short slender curved laminae, while in many genera and even families, such as the Productidoe, Stropkomenidoe, Lingulidoe, Disdnidoe, <fcc., there exists no calcified support for the labial appendages. The ventral valve in many of the genera is provided with two curved hinge-teeth, which fit into corresponding sockets in the opposite valve, so that the valves cannot be separated without breaking one of the teeth. Nearly all the genera composing the division Clistenterata have their valves articulated, while those forming the Tretenterata have theirs kept in position by the means of muscles especially adapted to that purpose; but in one of the most natural groups, viz., that of the Productidoe, we find genera presenting both conditions.

The intimate structure of the shell has been minutely investigated by Dr Carpenter, Prof. King, Dr Gratiolet, and several others, and been found to be distinct from that of the Lamellibranchiata and Gasteropoda. Dr Carpenter informs us that there is not in the shell of the Brachiopoda that distinction between outer and inner layers, either in structure or mode of growth, which prevails among the ordinary bivalves; that it seems obvious, both from the nature of the shell substance and from the mode in which it is extended, that the whole thickness of the Brachiopod shell corresponds with the outer layer only of the Lamellibranchiata; and that he has occasionally met with a second layer in recent Terebratuloe, within the earlier portion of the shell, but confined to only a part of the surface instead of extending beyond it. In some families composing the Clistenterata it consists, according to Prof. King, of three divisions, the innermost and middle ones, which constitute the entire thickness of the valve, being calcareous with a prismatic or fibrous structure, while the outer divisions would consist of a very thin mcanbrane. The innermost and intermediate divisions are in some families traversed by minute tubular canals, which pass from one surface to the other, for the most part in a vertical direction, and at tolerably regular intervals, but just before terminating near the outer surface of the epidermis their orifices suddenly become dilated, the lower half of the canals being often considerably smaller in diameter than the upper half. The canals are occupied by caecal processes proceeding from the mantle or the fleshy covering of the animal. Their function is, according to Dr Carpenter, branchial or subservient to respiration; but if there exists an outer epidermis, as described by King, which covers their expanded terminations, there would be no communication between the surrounding sea water and the mantle. In the Bhynchonellidae and in some other families the shell structure would, according to Dr Carpenter, consist of flattened prisms of considerable length, arranged parallel to each other with great regularity, and obliquely to the surface of the shell, the interior of which is imbricated by their outcrop. In certain genera, such as Lingtιla and Discina, no canals traverse the shell from the inner to the outer surface. The shell structure, according to Dr Gratiolet, would consist of two distinct elements, that is to say, a corneous or horny animal substance, and a testaceous one; these occur in alternate layers of unequal thickness. The testaceous layers recall the structure observable in the Terebratulïdae, being traversed by numerous canals of extreme or microscopic minuteness. As Mr Woodward observes in his excellent manual of the Mollusca, Prof. Huxley has suggested that the caeca are analogous to the vascular processes by which in many Ascidians the tunic adheres to the test, the extent of which adhesion varies in closely allied genera. It seems, however, strange that these tubular perforations should not have been essential to the species of every family composing the class if they are really subservient to respiration. The subject will therefore demand further consideration.

The anatomy of the Brachiopoda has been the subject of elaborate investigations by Cuvier, Vogt, Huxley, Hancock, Gratiolet, Woodward, Deslongchamps, King, and others, while of late years much light bas been likewise thrown on the embryology and early stages of the groups by Steenstrup, Lacaze-Duthiers, Morse, F. Müller, Oscar Schmidt, M t Crady, Kowalevsky, and others. Some differences in opinion, it is true, have been and still are entertained with respect to the exact function to be attributed to certain parts of the animal, but on all essential questions there is a pretty general agreement.

According to Morse the Brachiopoda are reproduced by eggs, generally kidney-shaped and irregular, which are discharged from the anterior margin of the shell, and drop just beyond the palliai membrane, hanging in clusters from the setae. Some uncertainty has prevailed as to whether there is a male and female individual. Lacaze-Duthiers and Morse state that the sexes are separate, and describe them as such in Thecidium and 7ereδraiWwια, and the French zoologist goes so far as to suggest that a difference is even observable in the shell, but the statement requires verification. Prof. Morse describes the embryo of Terebral·ulina with great minuteness during its six stages of development. It is divided into two, three, or four lobes clothed with vibratile cilia, and before becoming attached swims or whirls head foremost by means of vibratile cilia which cover the body. The same distinguished American zoologist describes with equal care the formation of the shell from its first stage of development to the adult condition. Lacaze-Duthiers alludes to two and four eye spots in the embryo of Thecidium, and states that the animal appears to be in some measure sensible to light.

The mouth conducts by a narrow oesophagus to a simple stomach which is surrounded by a large granulated liver. Owen’s “hearts” have been found to be oviducts, while the true heart consists of a pyriform vesicle appended to the dorsal surface of the stomach. The digestive organs and viscera, as well as the muscles, which take up only a small place in the neighbourhood of the beak, are separated from the great anterior cavity, and protected by a strong membrane in the centre of which the mouth is situated. The nervous system consists of a principal ganglion of no great size.

Both valves are lined by a delicate membrane termed the pallium or mantle; it secretes the shell, and is generally fringed with horny bristles or set®. It is composed of an outer and inner layer, between which are situated the blood channels or lacunes; in fact, all the internal parts of the shell are lined by the inner layer of the mantle, with the exception of the spots where the muscles attach themselve3 to the shell. The outer layer lines closely the inner surface of the valves to which it adheres, and in those species in which the shell is traversed by canals there exist, on the surface of the mantle facing the inner surface of the valves, corresponding short cylindrical membranous projections or lacunes, which insert themselves into the small tubular orifices that traverse the shell. The csecal prolongations do not exist in those genera, such as Rhynchonella, where the shell is deprived of tubular perforations. The inner layer is rather thicker than the opposite one, and is covered with vιbratile cilia. As stated by Nicholson and other anatomists, the blood channels form a remarkable system of more or less branched tubes, anastomosing with one another, and ending in the caec.al extremities. This, which has been termed by Huxley the arterial system, communicates with the perivisceral cavity by means of two or four organs, which are called pseudo-hearts, and which were at one time supposed to be true hearts. Each pseudoheart is divided into a narrow, elongated external portion (the so-called “ventricle”), which communicates, as Hancock has proved, by a small apicial aperture with the palliai cavity; and a broad, funnel-shaped so-called “auricle,” communicating on the one hand by a constricted neck with the so-called “ventricle,” and on the other by a wide patent mouth with a chamber which occupies most of the cavity of the body proper, and sends more or less branched diverticula into the palliai lobes (Huxley). The channels vary in their dispositions and details in different genera, and as they project to some small extent, leave corresponding indentations on the inner surface of the shell, so that their shape and directions can very often be traced on fossil and extinct genera as well as if the animal was still in life; this may be seen in the numerous illustrations appended to Davidson’s and other authors’ works treating of fossil Brachiopoda. There are usually four principal arterial trunks in each lobe of the mantle; the two central ones run direct to the front, near to...