422 PALLIOBRANCHIATA ; TEREBRATULIDJE. 



which serves for the support of the fringed spiral arms already 

 alluded to. The body of the animal, containing the viscera, 

 occupies but a small portion of the interior of the shell, and is 

 placed quite at the hinder portion of this cavity. In most cases 

 the intestine, which is convoluted, has no anal aperture. These 

 Mollusks have generally been described as possessing two or four 

 hearts, but according to the recent researches of Professor Hux- 

 ley, and Mr. A. Hancock, the organs which have been indicated as 

 hearts open into the cavity of the mantle, and probably serve for 

 the passage of the ova. The true heart is a pyriform organ at- 

 tached to the stomach. The circulation of the blood is effected 

 by a complicated system of vessels and lacunae, permeating 

 all parts of the body and mantle ; and there is also a system of 

 aquiferous canals, communicating with the chamber enclosing the 

 viscera, into which the water probably enters through the organs 

 above alluded to as having been regarded as hearts. 



1050. The greater part of the existing Mollusks included in 

 this Class, belong to the Family TEREBRATULIDVE, of which about 

 fifty species are at present known to exist, but of which several 

 hundred fossil species have been enumerated. These present 

 themselves in the very oldest rocks ; and may be found, in 

 greater or less abundance, in almost all marine deposits, down to 

 the present time. They possess the peculiar characters above 

 described in the fullest perfection, the two valves are unequal, 

 and usually united by a toothed hinge ; the lower valve is large, 

 convex, beaked, and perforated at the beak for the passage of the 

 peduncle ; and the upper valve is provided with a slender frame- 

 work of shelly substance for the support of the arms. This 

 framework is often found in fossil shells, in a beautiful state of 

 preservation. The arms of the ordinary Terebratulse do not ap- 

 pear to be extensible beyond the shell ; but in the Rhynchonella 

 psittacea they are enormously developed ; and being quite free 

 except at their origin, they may be extended far beyond the shell. 

 When drawn in, they are disposed in six or se v< n spiral turns. 

 The mechanism by which these arms are said to be unfolded is 

 very curious. The stem of each is tubular, and contains a 

 fluid, which, being acted upon by muscles forming the walls of 



