82 



MilNUAL OF THE MOLLUSCi 



pevuieated by mineral water, wkicli slowly deposits calcarious spar, in crys- 

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

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

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

 tions of orthocerata, it is evident 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 Kning 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 

 V.J) 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 uuwaiy crabs and shell-fish, 

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



The tetrabranchs could undoubtedly swim, by their respirator}' jets ; but 

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

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

 held a nearly vertical position, head-do^iiwards, 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 wdth the water.* The object 

 of the numerous partitions is not so much to sustain the pressm^e 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 INTr. Searles Wood) 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 

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

 v/ith the orthocerata, whose long straight shells were particularly exposed to 

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

 ci-eased size and strength of the siphuncle, and its increased vascularity. In 

 endoceras we find the siphuncle 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 large in the nautili, and causes an 



* A nautilus pompilius (in the cabinet of Mr. ^loiris) weighs lib., and -when the 

 siphuncle is secured, it floats witli a ^Ib weight in its aperture. The animal would 

 have displaced_2 pints (= 2pbs ) of water, and therefore, if it weighed 31bs., the specific 

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



t The siphuncle and lobed se])ta did not hold the animal in its shell, as Von Brch 

 imagined: that was secured by the shell-mu.<cles. The complicated sutures perhaps 

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

 eggs. 



