840 THE EQUIANGULAR SPIRAL [ch. 



two valves is stunted, and the growth of the other is (relatively 

 speaking) unopposed. 



Of septa 



Before we leave the subject of the molluscan shell, we have still 

 another problem to deal with, in regard to the form and arrangement 

 of the septa which divide up the tubular shell into chambers, in 

 the Nautilus, the Ammonite and their allies. 



The existence of septa in a nautiloid shell may probably be 

 accounted for as follows. We have seen that it is a property of 

 a cone that, while growing by increments at one end only, it con- 

 serves its original shape : therefore the animal within, which (though 

 growing by a different law) also conserves its shape, will continue 

 to fill the shell if it actually fills it to begin with: as does a snail 

 or other Gastropod. But suppose that our mollusc fills a part only 

 of a conical shell (as it does in the case of Nautilus) ; then, unless 

 it alter its shape, it must move upward as it grows in the growing 

 con^, until it comes to occupy a space similar in form to that which 

 it occupied before: just, indeed, as a little ball drops far down into 

 the cone, but a big one must stay farther up. Then, when the 

 animal after a period of growth has moved farther up in the shell, the 

 mantle- surface continues or resumes its secretory activity, and that 

 portion which had been in contact with' the former septum secretes 

 a septum anew. In short, at any given epoch, the creature is not 

 secreting a tube and a septum by separate operations, but is secreting 

 a shelly case about its rounded body, of which case one part appears 

 as the continuation of the tube, and the other part, merging with it 

 by indistinguishable boundaries, appears as the septum*. 



The various- forms assumed by the septa in spiral shells | present 

 us with a number of problems of great beauty, simple in their 

 essence, but whose full investigation would soon lead us into difficult 

 mathematics. 



* "It has been suggested, and I think in some quarters adopted as a dogma, 

 that the formation of successive septa [in Nautilus] is correlated with the recurrence 

 of reproductive periods. This is not the case, since, according to my observations, 

 propagation only takes place after the last septum is formed"; Willey, Zoological 

 Results, 1902, p. 746. 



f Cf. Henry Woodward, On the structure of camerated shells, Pop. Sci. Rev. 

 XI, pp. 113-120, 1872. 



