VARIETIES OF OYSTERS. 133 



The arms are frequently supported upon a calcareous 

 framework or skeleton, attached to the interior of the dor- 

 sal valve ; this usually forms two loops, springing from the 

 neighbourhood of the hinge. The arms appear to be 

 extensible in some instances ; but in others they are 

 attached to the internal framework, and only the extremity 

 is free. 



The body of the animal only occupies a small portion 

 of the cavity of the shell close to the hinge ; it is enclosed 

 within a strong membranous partition, in the centre of 

 which the mouth is seen. The intestine is convoluted, 

 and the liver is large and granular. 



All the Brachiopoda appear to possess two hearts, 

 -each composed of an auricle and a ventricle, situated in the 

 neighbourhood of the oesophagus ; they are also furnished 

 with a complex system of vessels, w r hich conveys the blood 

 to the organs of the body, and to the mantle, where it is 

 exposed to the influence of the water. 



The structure of the shell is very peculiar. It consists 

 of flattened prismatic cells, arranged in an oblique direc- 

 tion as regards the surfaces of the shell ; the substance of 

 the shell is traversed by small canals, through which little 

 processes of the outer layer of the mantle pass. 



These animals are all marine ; they are found attached 

 by the peduncle, which passes through the aperture in the 

 ventral valve, suspended from rocks, corals, and other 

 submarine objects. The fossil species are exceedingly 

 numerous, especially in the older strata, in some of which 

 they are the principal representatives of the Molluscous 

 type of structure. 



