to the successive stages of growth of, the outer shell, maintains its efficacy as a float, enlarging gradually and periodically until the animal has arrived at full maturity.[1]
A fifth consideration is had of mechanical advantage, in disposing the Distance at which these successive transverse Plates are set from one another. (See Pl. 31. Fig. 1. and Pl. 32, Fig. 1, 2.) Had these distances increased in the same proportion as the area of the air chambers, the external shell would have been without due support beneath those sides of the largest chambers, where the pressure is greatest: for this a remedy is provided in the simple contrivance of placing the transverse plates proportionally nearer to one another, as the chambers, from becoming larger, require an increased degree of support.
Sixthly, The last contrivance, which I shall here notice, is that which regulates the ascent and descent of the animal by the mechanism of the Siphuncle. The use of this organ has never yet been satisfactorily made out; even Mr. Owen's most important Memoir leaves its manner of operation uncertain; but the appearances which it occasionally presents in a fossil state, (see Pl. 32, Fig. 2, 3. and Pl. 33,)[2] supply
- ↑ In a young Nautilus Pompilius in the collection of Mr. Broderip, there are only seventeen Sepia. Dr. Hook says that he has found in some shells as many as forty. A cast, expressing the form of a single air chamber, of the Nautilus Hexagonous is represented in Pl. 42, Fig. 1.
- ↑ Pl. 32, Fig. 2, represents a fractured portion of the interior of a Nautilus Hexagonus, having the transverse plates (c. c'.) encrusted with calcareous spar; the Siphuncle also is similarly encrusted, and distended in a manner which illustrates the action of this organ. (Pl. 32, Fig. 2, a. a1. a2. a3. d. e. f, and Fig. 3, d. e. f.) The fracture at Fig. 2, b. shows the diameter of the siphuncle, where it passes through a transverse plate, to be much smaller than it is midway between these Plates (at d. e. f.) The transverse sections at Fig, 2 a. and h., and the longitudinal sections at Fig. 2, d. e. f., and Fig. 3, d. c. £, show that the interior of the siphuncle is filled with stone, of the same nature with the stratum in which the shell was lodged. These earthy materials, having entered the orifice of the pipe at a in a soft and plastic state, have formed a cast which shows the interior of this pipe, when distended, to have resembled a string of
