we have a singular compound oceanic Tunicate, in which the numerous zooids form a tubular colony, which is propelled through the water by the united excurrent respiratory jets of its component members. Like the Salpians, it is brilliantly phosphorescent.

On the other hand, the more typical Tunicates are found attached to all kinds of submarine objects, or (as in Pelonaia) imbedded in mud.

No Tunicates are known with certainty to have been preserved in the fossil condition, but the singular Silurian genus Pasceolus has been doubtfully referred to this class.

CHAPTER XLIV.

BRACHIOPODA.

CLASS III.-BRACHIOPODA (Palliobranchiata).-The members of this class are defined by the possession of a body protected by a bivalve shell, which is lined by an expansion of the integ ument, or mantle." The mouth is furnished with two long cirriferous arms. The nervous system consists of a single ganglion, placed in the re-entering angle between the gullet and the rectum, so that the intestine has a neural flexure."

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The Brachiopoda are essentially very similar in structure to the Polyzoa, from which they are distinguished by the fact that they are never composite, and by the possession of a bivalve, calcareous, or sub-calcareous shell. They are commonly known as "Lamp-shells," and are all inhabitants of the sea. All the living forms, except Lingula pyramidata, are fixed to some solid object in their adult condition; but there is good reason to believe that many of the fossil forms were unattached and free in their fully-grown condition. From the presence of a bivalve shell, the Brachiopods have often been placed near the true bivalve Mollusca (the Lamellibranchiata), but their organisation is very much inferior, and there are also sufficient differences in the shell to justify their separation.

The two valves of the shell of any Brachiopod (figs. 185, 187) are articulated together by an apparatus of teeth and sockets, or are kept in apposition by muscular action alone. One of the valves is always slightly, sometimes greatly, larger than the other, so that the shell is said to be "inequivalve." As regards the contained animal, the position of the valves is

anterior and posterior, so that they are therefore termed respectively the "ventral" and "dorsal" valves. In the ordi

Fig. 185.-Rhynchonella sulcata. A, Profile view. B, View of the dorsal surface. C, View of the base. a Ventral valve; 6 Dorsal valve; Base; c Beak; h Foramen. Lower Cretaceous.

nary bivalve Mollusca (Lamellibranchiata), on the other hand, the two valves of the shell are usually of the same size (equivalve), and they are situated upon the sides of the

Fig. 186.-Morphology of Brachiopoda. A, Lingula pyramidata (after Morse); Peduncle; s Sand-tube, encasing base of peduncle. B, Lingula anatina (after Cuvier); The peduncle. C, Waldheimia cranium, with adherent young, attached to a stone (after Davidson); Peduncle; v Ventral valve; d Dorsal valve. D, Crania Ignabergensis, attached by its ventral valve to a piece of coral (Chalk).

in the shell of the Brachiopoda is usually the largest, and usually possesses a prominent curved beak. The beak (figs. 185, 187) is often perforated by a "foramen," or terminal aperture, through which there is transmitted a muscular peduncle, whereby the shell is attached to some foreign object. In some cases, however (as in Lingula, fig. 186), the peduncle simply passes between the apices of the valves, and there is no foramen; whilst in others (as in Crania, fig. 186, D) the shell is merely atttached by the substance of the ventral valve. The dorsal or smaller valve is always free, and is never perforated by a foramen.

In intimate structure, the shell of most of the Brachiopoda consists "of flattened prisms, of considerable length, arranged parallel to one another with great regularity, and at a very acute angle-usually only about 10° or 12-with the surfaces of the shell" (Carpenter). In most cases, also, the shell is perforated by a series of minute canals, which pass from one surface of the shell to the other, in a more or less vertical direction, usually widening as they approach the external surface. These canals give the shells a "punctated" structure, and in the living animal they contain cæcal tubuli, or prolongations, from the mantle, which are con sidered by Huxley as analogous to the vascular processes by which in many Ascidians the muscular tunic, or "mantle," is attached to the outer tunic, or "test." In some of the Brachiopoda (as in the Rhynchonelliaz) the shell is "impunctate," or is devoid of this singular canal system.

Though characteristically calcareous, the shell of the Brachiopoda may sometimes be largely composed of horny matter (as in Discina); or the carbonate of lime in the horny shell may be almost wholly replaced by phosphate (as in Lingula).

The inner surface of the valves of the shell is lined by expansions of the integument which secrete the shell, and are called the "lobes" of the "pallium," or "mantle." The digestive organs and muscles occupy a small space near the beak of the shell, which is partitioned off by a membranous septum, which is perforated by the aperture of the mouth. The remainder of the cavity of the shell is almost filled by two long oral processes, which are termed the "arms," and from which the name of the class has been derived (fig. 187, D). These organs are lateral tubular prolongations of the margins of the mouth, usually of great length, closely coiled up, and fringed on one side with ciliated lateral processes, or "cirri." In many Brachiopods the arms are supported upon a more or less complicated internal calcareous framework or skeleton, which is sometimes called the "carriage-spring apparatus," and which in many extinct forms is coiled into a shelly spiral.

The mouth conducts by an oesophagus into a distinct stomach, surrounded by a well-developed granular liver. The intestine has a "neural flexure," and "either ends blindly in

the middle line, or else terminates in a distinct anus between the pallial lobes" (Huxley).

Within the pallial lobes there is a remarkable system of

Fig. 187.-Terebratula (Waldheimia) flavescens. A, The shell viewed from behind, showing the dorsal valve, and the perforated summit of the ventral valve above it. B, Inner view of the dorsal valve, showing the shelly loop () which supports the spiral arms. C, Inner view of the ventral valve, showing the foramen or aperture (ƒ) in the beak, through which the muscular stalk of attachment passes. D, Longitudinal and vertical section of the animal, showing the spiral arms (a), the stomach (s), and the liver (h). At is the opening in the beak, with the stalk of attachment () passing through it. (After Davidson and Owen.) Some details have been omitted in figs. B, C, and D, for the sake of clearness.

more or less branched excretory tubes, anastomosing with one another, and ending in cæcal extremities. This, which has been termed by Huxley the "atrial 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, but which are now known to be connected with reproduction.

"Each pseudo-heart is divided into a narrow, elongated, external portion (the so-called 'ventricle'), which communicates, as Dr Hancock has proved, by a small apical aperture, with the pallial cavity; and a broad, funnel-shaped, inner division (the 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 pallial lobes " (Huxley). This system of the atrial canals has been looked upon as a rudimentary respiratory apparatus; but its function is more probably to act as an excretory organ, and it certainly serves also to convey away the reproductive elements, the organs for which are developed in various parts of its walls. By Rolleston the pseudo-hearts are looked upon as corresponding with the so-called "organ of Bojanus " of the Lamellibranchiata.

The function of respiration is probably performed, mainly, if not entirely, by the cirriferous oral arms, as it appears chiefly to be by the homologous tentacular crown of the Polyzoa. A

Fig. 188.-Development of Terebratulina septentrionalis (after Morse). A, Ciliated embryo. B, More advanced embryo, showing commencing segmentation and a rudimentary peduncle (p). C, D, E, F, Further stages of the same embryo. G, Advanced embryo, with a very long peduncle (), and a circular oral crown of cirri (c) H, Interior of dorsal valve, showing the circular crown of cirri, and the intestine (4). I, Another larva, at the same stage, having the valves opened, and viewed from one side. J, Part of a larva still further advanced, showing the now horse-shoe-shaped crown of cirri; Peduncle; v Ventral valve of shell; d Dorsal valve; c Crown of oral citri; i Intestine; s Setæ springing from the edge of the mantle; / Loop of dorsal valve. (All the figures are highly magnified.)

unilocular heart is present in some, but apparently not in all, of the Brachiopoda; and the circulation seems to be mainly carried on through the interstices between the tissues.

The nervous system consists of a principal ganglion of no great size, placed in the re-entering angle between the gullet and the rectum. In those Brachiopods in which the valves of the shell are united by a hinge, the nervous system attains a