TUNICATA, BRACIIIOPODA. 277 



Ascidians, of which the solitary young gives origin, by gemmation, 

 to a compound group, which again procreates, by impregnated ova, 

 solitary individuals. 



Class Brachiopoda. 



These Acephalous Mollusca are deprived, like the Ascidians, of the 

 power of locomotion, and are attached by a longer or shorter peduncle 

 to foreign bodies. Their muscular tunic or mantle is, as it wei'e, slit 

 open, and consists of two broad membranous expansions, called lobes, 

 which are covered by two calcareous plates, adapted to inclose and 

 defend all the soft parts of the animal. 



The Brachiopoda flourished during the ancient secondary periods, 

 and are most abundant in the fossil state. Of the few existing genera, 

 he Lingula is characterised by its long peduncle, and the equality of 

 the valves of its shell, neither of which are perforated : the Orbiciila 

 is sessile, and adheres by one end of a short transverse muscle, which 

 perforates one of the valves of the shell : the Terebratula is attached 

 by a short peduncle, which projects through a hole in a beak-shaped 

 prolongation of one 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 ciliated arms, continued from the sides of the 

 mouth, and disposed in folds and spiral curves. 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, 

 which opens upon 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 turned towards the dorsal valve : the bent portions of 

 the fringed arms are supported by slender and elastic calcareous \)yo- 

 cesscs. Remains of more complicated internal calcareous appendages 

 are presented by certain extinct Brachiopods, as the Spirifer. In 

 some species of existing Terebratula, as Ter. psittacea, the arms are 

 disposed in a series of spiral folds; but they have only short and 

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

 OrbiculcB and Linguloi have no internal calcareous support. In all 

 the Brachiopods the stem which supports the brachial fringe is 



T 3 



