82 



MANUAL OF THE MOLLUSCA. 



peiineated by mineral water, which slowly deposits calcarious spar, in C17S- 1 



tais, on their walls ; or by acidulous water, which removes eveiy trace of the i 



sheU, leaving a ca\dty, which at some future time may again become filled j 



\dth spar, having the form of the shell, but not its structm-e. In some sec- I 



tions of orthocerata, it is evident that the mud has gained access to the air- ! 

 cells, along the com'se of the blood-vessels ; but the chambers are not entirely 



tilled, because their lining membrane has contracted, leaving a space between j 



itself and certain portions of the walls, which correspond in each chamber. | 



With respect to the purpose of the air-chambers, much ingenuity has j 



been exercised in devising an explanation of their assumed hydrostatic func- ] 



tion, whereby the nautilus can rise at will to the surface, or sink, on the j 



approach of storms to the quiet recesses of the deep. Unfortimately for such j 



poetical speculations, the nautilus appears on the surface, only ichen driven \ 



up by storms, and its sphere of action is on the bed of the sea, where it j 



creeps like a snail, or perhaps lies in wait for unwary crabs and sheU-fish, \ 



lilce some gigantic " sea-anemone," with outspread tentacles. j 



The tetrabranchs could undoubtedly swim, by then- respii-atory jets ; but i 

 the discoidal nautili and ammonites are not well calculated, by their forms, ■■ 

 for s\\-imming ; and the straight -shelled orthocerata and bacidites must have { 

 held a nearly vertical position, head-downwards, on account of the buoyancy ' 

 of their shells. The use of the air-chambers, is to render the whole animal ; 

 (and shell) of nearly the same specific gravity ^ith the water.* The object ! 

 of the numerous partitions is not so much to sustain the pressure of the water, i 

 as to guard against the collisions to which the shell is exposed. They are most \ 

 complicated in the ammonites, whose general form possesses least strength.! 

 The jmrpose of the siphuncle (as suggested by Mr. Searles Wood) is to main- ) 

 tain the vitality of the shell, during the long life which these animals cer- ! 

 taiidy enjoyed. INIr. Forbes has suggested that the inner com-ses of the 1 

 hamites, broke off, as the outer ones were formed. But this was not the case ! 

 ^\dth the orthocerata, whose long straight shells were particidarly exposed to j 

 danger ; in these the preservation of the shell was pro\dded for by the in- 

 creased size and strength of the siphuncle, and its increased vascularity. In [ 

 endoceras we find the siphimcle thickened by internal deposits, until (in some ( 

 of the very cylindrical species) it forms an almost solid axis. 



The nucleus of the shell is rather lai-ge in the nautili, and causes an I 



* A nautilus pompilius (in the cabinet of Mr. Morris) weighs lib., and when the i 



siphuncle is secured, it floats with a |lb weight in its aperture. The animal would ! 



have displaced 2 pints (= 2ilbs ) of water, and therefore, if it weighed 31bs., the specific 1 



gravity of the animal and shell would scarcely exceed that of salt water. 1 



t The siphuncle and lobed septa did not hold the ariimal in its shell, as Von Buch 'i 



imagined: that was secured by the shell-muscles. The complicated sutures perhaps ;j 



indicate lobed ovaries; they occur in genera, which must have produced very small 1 

 eggs. 



