28 THE SHELL. 



mantle, with an underlying spongy calcareous mass attached. 

 In Sepia we find always, the so-called aerial chambers obliquely 

 placed and not connected by a siphon, and sometimes terminated 

 by a sharp rostrum, whilst in fossil genera, as Beloptera, these 

 chambers are arranged in a single line, or in Spirulirostra (xxix, 

 81), they become a spiral series, connected by a siphon and 

 analogous to the shell of the Spirula (xxvi, 53), which latter lies 

 free in the mantle, without the envelopment of a spongy mass. 

 In another group of fossil forms, the long shell is composed of 

 a narrow or broad anterior corneous portion, and a posterior 

 calcareous part containing the aerial chambers, placed one upon 

 another and siphunculated. These chambers are only covered 

 with shell in Conoteuthis ; but they are protected in the Beleni- 

 nites by a testaceous rostrum (ii, 19); sometimes very long, 

 which, absolutely identical with that of Sepia, is composed of 

 successive very compact radiating layers. 



The internal shell, in relation to the animal economy, demands 

 some further consideration. These functions, by reason of 

 modifications of structure, are threefold : 



1. If it is a corneous blade, it becomes simply a support to 

 the flesh, fulfilling that oflSce of the skeleton in mammals. 



2. When it is corneous or testaceous, and containing parts 

 filled with air, as in the alveola of the Belemnites, it, perhaps, 

 additionally represents among mollusks the swimming-bladder 

 of fishes. These air-chambers may consist, as we have seen, of 

 an oblique series, separated in their interior by a crowd of small 

 diaphragms, filled with air, and attached to the under side of the 

 blade or cuttle-bone, as in Sepia ; or even of a series of chambers 

 taking a definite spiral form, as in Spirula. D'Orbigny shows 

 that shells of this second division, when parted from their 

 animals, are sufficiently light to float upon the surface of the 

 waves, and that there is a constant coincidence of the progres- 

 sive augmentation of the number of air-chambers with the 

 growth of the animal, in order to maintain an equilibrium. The 

 lightness of the shell of the Sepia appears to be partly due to a 

 contained gas, which Dr. Paul Bert has succeeded in obtaining 

 in small quantities, by opening the sack of the animal under 

 water. In efi'ect, the Sepia and the Spirula, animals of massive 

 proportions, have need of this aid in swimming ; and it is more 

 plentifully supplied to the round-bodied Spirula, than to the 

 Conoteuthis, for example, the form of which denotes an animal 

 infinitely more agile. In the Belemnites the aerial chambers 

 doubtless compensated the enormous weight of the calcareous 

 rostrum, which would otherwise have compelled the animal to 

 maintain a vertical position in the water, or prevented horizontal 

 movement, except at great disadvantage to its strength. (In 

 the chambered external shells of the tetrabranchiates, represented 



