in Cephalopoda [Sij^honopoda) . 301 



as I will now prove. True that in old age the septa of iVaw- 

 ?e7?<* gradually approximate; further, it should seem proven 

 that the septa are directly formed by the animal [sc. by secre- 

 tion of some sort), and that there is no subsequent growth of 

 the intraseptal zone of shell-wall. What, then, does this 

 approximation of septa mean? Primarily it is caused by 

 diminution of body-growth ; this renders it less necessary for 

 the moUusk to move into a wider portion of its shell ; it there- 

 tore advances more slowly. Nevertheless the formation of 

 sej)ta does not slacken ; they are formed at the same intervals 

 of time as usual ; the mtervals of space between such septa 

 are therefore less than those between septa formed when the 

 animal was more rapidly growing. None can suppose that 

 the formation of septa at these shorter distances is intended 

 to benetit the aged animal, as though, compelled to advance, 

 it were unable to do so without support ; the very closeness 

 of the septa proves the contrary. Thus, in many fossil 

 specimens of Ammonoidea and Nautiloidea, the last-formed 

 septa are very closely pressed together. More forcible is the 

 evidence of a recent adult Nautilus-shell belonging to Prof. 

 Moseley ; here the last septum is twice as thick as that pre- 

 ceding ; indeed, a line down the middle shows that it is really 

 formed of two septa. It must have been a very feeble Nau- 

 tilus that could not advance a single millimetre without a 

 wall to lean its back against. We must therefore conclude 

 that this pathological approximation of septa in the chambered 

 shell of a senescent, or, more rarely, of a middle-aged Cepha- 

 lopod is due to the continued activity of the secretory function 

 alter the relaxation of the other functions. 



So far secretion of the shell is probable but not proven ; 

 further discussion will be found in the sequel. Riefstahl's 

 total negation is based chiefly on the compression of the last- 

 formed membranes in the sepion. We have therefore to 

 prove that the approximation of septa in Nautilus is not 

 homologous with the apparently similar compression of mem- 

 branes in ISepia. The argument follows two lines — (a) Phy- 

 logenetic, (/3) Ontogenetic. 



(a) Granted that abnormal approximation of septa is due to 

 the persistence of septal formation when the other wheels of life 

 run down or are thrown for the time out of gear, we shall see 

 that this feature has for the evolutionist a wider meaning*. 

 The characters of old age, produced as they are by failure of 

 powers, are often curiously simulated by the symptoms of a 

 disease that is not " specitic," but due to overwork of the 

 organism. Thus the geratological * character of approaching 

 Geratohifjy, the science of old age : Hyatt. 

 Ann. cfe Mag. N. Hist. !Ser. 6. Vol. i. 21 



