486 LECTURE XX. 



teeth, developed from the margin of the ventral valve, and received 

 by sockets in the other : the joint permits only a very slight divari- 

 cation of the valves. 



The viscera are situated at the part of the shell next the hinge or 

 peduncle, and are confined to a very small space in the Terebratula. 

 The rest of the interspace of the lobes of the mantle is almost entirely 

 occupied by two long fringed arms, continued from the sides of the 

 mouth, and disposed in folds and spiral curves (m). The bases of 

 the arms are confluent, and form a transverse fringed band above 

 the mouth : a narrow parallel fold of membrane passes below the 

 mouth (w), which opens upon or towards the mantle-lobe attached to 

 the perforated valve. 



In the Terebratula australis each arm extends outwards, advances 

 forwards, curves slightly inwards, and bends abruptly back upon 

 itself, the two parts of the bend being connected together ; then the 

 stem again curves forward, and becomes united to the corresponding 

 bend of the opposite arm, the conjoined extremities describing spiral 

 convolutions ; the bent portions of the fringed arms are supported by 

 the slender and elastic calcareous loop {h,fig. 187), which is bent 

 back upon itself (c). In the Ter. dorsata the advancing crura of 

 the loop, beside their basal origins, are attached by two slender 

 transverse bars to a median ridge of the valve. In Ter, caput 

 serpentis the crura are united by a slender arch, half way between 

 their bases and the loop, which is short and not reflected. In The- 

 cidium the calcareous loop is folded into two or more lobes, lying in 

 hollows of the dorsal valve. In Rhynchonella the apophysary system 

 is reduced to two short flattened and grooved lamellae attached to the 

 inner side of the beak of the dorsal valve. Remains of more com- 

 plicated internal calcareous appendages are presented by certain 

 extinct Brachiopods, as the Spirifer. In some species of existing 

 Terebratulce, as Ter. (^Rhynchonella) psittacea, the arms are wholly 

 disposed in a series of spiral folds, supported by only the short and 

 simple calcareous processes at their base. Tiie spiral arms of the 

 OrbiculcB and LingulcB have no internal calcareous support. In all the 

 Brachiopods, the stem which supports the brachial fringe is hollow, 

 and the canal is a closed cavity in each arm. In the Terebratulce and 

 OrbiculcB the spiral terminations of the arms have the canal sur- 

 rounded by muscular fibres ; the canal is filled with fluid, and, by the 

 contraction of these fibres, the extremities are extended by the pres- 

 sure of the contained fluid which is injected into them. 



In most Terebratulidce the [shell is traversed by minute canals 

 from one surface to the other nearly vertically and at regular dis- 

 tances; their external orifices are slightly expanded. The adhesion 



