Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/232

190 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 hinge- plate, 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 Twebratulina becomes annular in the adult by the union of the oral processes or crura (fig. 16). In Waldheimia it is elongated and reflected ; the hinge- plate 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 Terelratella the loop is attached to the hinge-plate and to the septum (fig. 17). In Megerlia, it is three times attached, first to the hinge- plate, and then to the septum by processes from the diverging and reflected positions of the loop. In Mayas 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 lam elite, the lower ones riband-shaped ; attached first to the hinge-plate, they after wards 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 anchor- shaped lamellae (fig. IS). In Bouchardia the septum only is furnished with two short anchor-shaped lamellse. Many more modifications are observable in different groups of which the great family Terebratulidm 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 Spiriferidce there are two conical spires directed outwards, and nearly filling the cavity of the shell (fig. 5) ; while in Atrypa the broad spirally coiled lamellas are vertical, and directed towards the centre of the valve. In the llliyncliondlidw there are two short slender curved laminae, while in many genera and even families, such as the Productidce, Strophomenidce, Lingulidce, Discinidce, etc., 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 Prodactidce, we find genera presenting both conditions.

The intimate structure of the shell has been minutely structure, 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 Terebratulce, 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 niftinbrane. Tlio 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 caeca! processes proceed ing 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 ex panded terminations, there would be no communication between the surrounding sea water and the mantle. In the Rhynchonellidas 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 lAngida 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 Terebratididce, 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 arc 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 has been likewise thrown on the embryology and early stages of the groups by Steenstrup, Lacaze-Duthiers, Morse, F. Miiller, Oscar Schmidt, M Crady, Kowalevsky, and others. Some differences in opinion, it is true, have been and still arc entertained with respect to the exact function to be attri buted 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 arc discharged from the anterior margin of the shell, and drop just beyond the pallial membrane, hanging in clusters from the setie. 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 Terebratulina, and the French zoologist goes so far as to suggest that a differ ence is even observable in the shell, but the statement requires verification. Prof. Morse describes the embryo of Terebratulina 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 distin guished 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. 