22 EXTERNAL SHELL. 



piliiiH in the cabinet of Mr. Morris weighs 1 lb., and when the 

 sii)hunele is secured it floats with a half-i'iound weight in its 

 aperture. The animal would have displaced two pints or 2-.5 lbs. 

 of water, and, therefore, if it weighed 3 lbs., the si)ecific gravity 

 of the animal and shell Avould scarcely exceed that of salt Avater. 

 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. The 

 complicated sutures perhaps indicate lobed ovaries; they occur 

 in genera which must have ])rodueed very small eggs. The pur- 

 pose of the siphuncle (as suggested by Mr. Searles Wood) is to 

 maintain the vitality of the shell during the long life which these 

 animals certainly enjoyed. Mr. Forbes has suggested that the 

 inner course of Hamites broke off as the outer ones were formed. 

 But this was not the case with the Orthocerata, Avhose long, 

 straight shells were particularly exposed to danger ; in tliese the 

 preservation of the shell was provided for by the increased size 

 and strength of the sipliuucle, and its increased vasculai'ity. 



In Endoceras we lind the siphuncle thickened by internal 

 deposits, until in some of the very cylindrical species it forms an 

 almost solid axis. It has been stated that the septa are formed 

 periodically; but it must not be supposed that the shell-muscles 

 ever become detaclied, or that the animal moves the distance of 

 a chamber all at once. It is most likely that the adductors 

 grow only in front, and that a constant waste takes place 

 behind, so that tliey are ahvnys moving forward, except when a 

 new septum is to l)e formed ; the sei)ta indicate periodic rests. 

 The consideration of this fact, that the Nautilus must so fre- 

 (piently liave an air-cavity l>etween it and its shell, is alone 

 suHicient to convince us that the chambered cephalopods could 

 not exist in very deep water, 'i'hey were i)r(»ltal)ly limited to a 

 depth of 20 or 30 fatiioms at Ihv utmost.* 



The specific gravity of the chambered shells of cephalopods 

 being such as to enable them to float upon tlie surface of the 



* The air-chambers would be crushed by the pressure of water at any 

 consideral>le depth : this pressure exceeding 265 lbs. to the square inch 

 at 100 fatlioins -at which depth, empty bottles, securely corked, are 

 crushed. 



