[056]

PLATE 34. V. I. p. 329. Note.

Drawing of the animal of the Nautilus Pompilius, prepared at my request by Mr. Owen, to shew the manner in which the siphuncle terminates in the Pericardium. (Original.) [057]

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 chamber.

d. d. d. d. Follicles communicating with the Branchial Arteries. *

'd. 'd. 'd. 'd. Pericardal septa, forming thin muscular
Receptacles of the follicles.

e. e. The Branchiæ.
f. The Branchial Chamber.
g. The Funnel, or Branchial outlet.
h. The infundibular valve.
i. i. The digital processes.
k. The Gizzard.
l. The Ovary.
m. m. The mantle dissected off.
n. The membranous siphuncle.
o. o. 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.

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* Mr. Owen supposes that these follicles discharge the impurities of the blood into the Pericardium, when there is no access of water to the Branchiæ, during the time that the animal is contracted within its shell. The overflowings of this pericardial fluid may pass out through the orifices marked by the bristles, c. c.

This upper portion or neck of the Siphuncle, has the form of a flattened canal, with thin Parietes of the same substance as the Pericardium when the animal expands itself at the bottom of the sea, this neck is probably closed by the lateral pressure of the gizzard, k, and ovary, l, 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 penicardial fluid into the Siphiincle; but as an equal amount or pressure is applied simultaneously to the Ovary and [058]  Gizzard, the lateral pressure of these two organs on the neck of the Siphuncle would tend to close it with a force exactly counterbalancing the external pressure on the Pericardium.