M MANUAL OF THE MOLLUSCA. 



permeated by mineral water, which slowly deposits calcarious spar, in crys- 

 tals, on their walls ; or by acidulous water, which removes every trace of the 

 shell, leaving a cavity, which at some future time may again become filled 

 ar, having the form of the shell, but not its structure. In some sec- 

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

 cells, along the course of the blood-vessels ; but the chambers are not entirely 

 filled, because their lining membrane has contracted, leaving a space between 

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



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

 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 

 approach of storms to the quiet recesses of the deep. Unfortunately for such 

 poetical speculations, the nautilus appears on the surface, only when driven 

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

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

 like some gigantic " sea-anemone." with outspread tentacles. 



The tetrabranchs could undoubtedly swim, by their respiratory jets 

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

 for swimming ; and the straight -shelled orthocerata and baculites must have 

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

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

 (and shell) of nearly the same specific gravity with the water.* The object 

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

 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.f 

 The purpose of the siphuncle (as suggested by Mr. Searles \Vood) is to main- 

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

 tainly enjoyed. Mr. Forbes has suggested that the inner courses of the 

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

 with the orthocerata, whose long straight shells were particularly exposed to 

 danger ; in these the preservation of the shell was provided for by the in- 

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

 endoceras we find the siphuncle thickened by internal deposits, until (in s 

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



The nucleus of the shell is rather large in the nautili, and causes 



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

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

 have displaced^ pints (= 2|lbs ) of water, and therefore, if it weighed 31bs., the spe< 

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



t The siphuncle and lobed septa did not hold the animal in its shell, as Von B- 

 imagined: that was secured by the shell-mu&cles. The complicated sutures perh 

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

 eggs. 



