'2'2 EXTERNAL SHELL. 



///7///S in the cabinet of Mr. Morris weighs 1 lb., and when the 

 siphuncle is secured it Moats with :i half-pound weight in its 

 aperture. The Mniinnl would have displaced two pints or 2-f> Ins. 

 of water, and, therefore, if it, writhed :j Ibs., the specific gravity 

 of the animal and shell would scarcely exceed thai of salt 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 .'ire most complicated in the 



Ammonites, whose general form possesses Least strength. The 



complicated BUtures perhaps indicate lobed ovaries; they occur 

 in genera which must have produced very small eggs. Tin- pur- 

 pose of the siphunele (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 oil' as the outer ones were formed. 

 But this was not the case with the Ort hocerata, whose long, 

 straight, shells were particularly exposed to danger; in these the 

 preservation of the shell was provided for by the increased si/e 

 and strength of the siphunclc, and its increased vascularity. 



In Undoceras we find the siphuncle thickened by internal 

 deposits, until iu some of the very cylindrical species if forms an 

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

 periodicallv : but it must not, be supposed that the shell-muscles 

 ever become detached, or that tin- animal moves the distance of 

 a chamber all at once. If is most likely that the adductor* 

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

 behind, so that they a. re always moving forward, except when a 

 new septum is to be formed ; the septa indicate periodic- rests. 

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

 .pienllv have an air-cavity between it and its shell, is alone 

 suHicient to convince us that the chambered cephalopods could 

 not exist in very deep water. They were probably limited to a 

 depth of -J(l or :JO fathoms at the iitm<>- 



The specific gravity >f t he chambered shells of cephalopods 

 beinir such as to enable themtolloat upon the surface of t he 



* The air-rhainlxM-s would lf rnislu'd ly tin 1 pressure of water at any 

 considerable depth : this pressure exceeding '.'(I.") ll).s. to the S(|u:uv inch 

 ;it KM) t'ntlioiiis ;it which depth, empty liollles, seciii'ely eoi'ked, :ire 



crushed. 



