184 MANUAL OF THE MOLLUSCA. 



calculated, by their forms, for swimming; and the straight- 

 shelled orthocerata and baculites must have held a nearly vertical 

 position, head downwards, on accotint 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 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 compli- 

 cated in the ammonites, whose general form possesses least 

 strength, t The purpose 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 the hamites broke off as 

 the outer ones were formed. But this was not the case with the 

 orthocemta^ whose long straight shells were particularly exposed 

 to danger ; in these the preservation of the shell was j)rovided 

 for by the increased 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 cylin- 

 drical species it forms an almost solid axis. 



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

 causes an opening to remain through the shell, until the umhilicus 

 is filled up with a callous deposit ; several fossil species have 

 always a hole through the centre. 



In the ammonites, the nucleus is exceedingly small, and the 

 whorls compact from the first. 



It has been stated that the septa are formed periodically ; but 

 it must not be supposed that the shell-muscles ever become 

 detached, 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 they 

 are always moving onward, except when a new septum is to be 

 formed ; the septa indicate periodic rests. 



The consideration of this fact, that the nautilus must so 

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

 sufficient to convince us that the chambered cephalopods could 



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

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

 have displaced two pints (= 2jlbs.) of water, and therefore, if it weighed 31bs., the 

 epeciflc 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 Buch 

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

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

 eggs. 



