EXPLANATION OF PLATE 34. 57 



a. The Heart. 



b. A bristle passing from the pericardium through the 



membranous siphuncle laid bare. 



c. Bristles passing from the pericardium through the 



orifices of communication with the Branchial cham- 

 ber. 



J. d. d. d. Follicles communicating with the Branchial 

 Arteries.^ 



V/. 'd. 'd. 'd. Pericardial septa, forming thin muscular 

 Receptacles of the follicles. 



e. e. The Branchiee. 



f. The Branchial Chamber. 



g. The Funnel, or Branchial outlet. 

 h. The infundibular valve. 



i. i. The digital processes. 

 k. The Gizzard. 

 /. The Ovary. 



m. m. The mantle dissected off. 

 n. The membranous siphuncle. 

 0. 0, The siphuncular artery. 

 p» p. The Boundaries of the Pericardial cavity. 

 q. Portion of the Siphuncle between the Pericardium and 

 first transverse plate of the shell.f 



* Mr. Owen supposes that these follicles discharge the impurities 

 of the blood into the Pericardium, when there is no access of water 

 to the Branchife, during the time that the animal is contracted witliin 

 its shell. The overflowings of this pericardial fluid may pass out 

 through the orifices marked by the bristles, c. c. 



f This upper portion or neck of the Siphuncle, has the form of a 

 flattened canal, with thin Parietes of the same substance as the Peri- 

 cardium ; when the animal expands itself at the bottom of the sea, 

 this neck is probably closed by the lateral pressure of the gizzard, A, 

 and ovary, Z, and so acts instead of a valve to prevent the return of 

 the pericardial fluid into the Siphuncle. At such times, the deep-sea 

 water must press with great force on the exterior of the Pericardium, 

 and tend to force the pericardial fluid into the Siphuncle ; but as an 

 equal amount of pressure is applied simultaneously to the Ovary and 



