362 



LOWER INVERTEBRATES. 



of its growth have not yet been studied. In the adult it is coiled in a flattened 

 spiral, like that of a Planorbis. A very important distinction is, however, to be 

 noticed in the relations of the shell to the animals in these two forms. In Planorbis 

 the foot is turned toward the inside of the coils ; in Nautilus the reverse is the case. 

 The shell, to exactly correspond, should be unrolled and coiled in the opposite direction. 

 Besides this peculiarity the shell of the Nautilus is chambered. At regular periods 

 of growth the animal moves forward in its shell, and partitions off a chamber by 

 means of a calcareous partition ; again and again is the process repeated until the 

 result is the series of cells so familiar to all in the sawed shell of the pearly nautilus. 



A similar partitioning off of the shell is somewhat common among the gasteropods, 

 and several instances have been mentioned in the j)receding pages, but in Nautilus 

 and its allies a feature is introduced unknown elsewhere in the Mollusca, and, indeed, 

 in the animal kingdom. In these forms the partitions of the chambered shell are 

 traversed by a cord-like pedicle arising from the central portion of the dorsal region of 

 the body. This pedicle, in passing through the partitions, forms a more or less tubular 

 series of openings known as the siphuncle, and by this tube any recent or fossil shell 

 may at once be recognized as belonging to the Cephalopoda. The deserted chambers 

 are filled in the living specimens with a gas, the purpose of which seems to be to lessen 

 the specific gravity of the animal. The gas is said to be a mixture of oxygen and 

 nitrogen, the latter in greater proportion than in the atmosphere. How it obtains 

 entrance tothe chambers is unknown; possibly it is secreted by the animal, as is the 

 case in many other animals. Arcella (one of the Protozoa allied to Difflugia) fills its 

 shell with gas ; the float of the siphonophores acquires its buoyant qualities from the 

 contained air, while in the vertebrates the gaseous contents of the air-bladder in the 

 bony fishes will at once suggest itself in this connection. 



The mode of the formation of the chambers and the forward move- 

 ment of the animal are also obscure. It would, however, appear prob- 

 able that there is a periodicity in the operations, and it has been 

 suggested that " each septum shutting off an air-containing chamber is 

 formed during a period of quiescence, probably after the reproductive 

 act, when the visceral mass of the Nautilus may be slightly shrunk, and 

 gas is secreted from the dorsal integument so as to fill up the space pre- 

 viously occupied by the animal." 



Between the shell of the Nautilus and that of other forms a con- 

 siderable range occurs, the study of which requires a reference to both 

 living and fossil forms. A series can be arranged from large shells like 

 that of the Nautilus, leading down through small and internal but still 

 chambered forms, like that of Spirula, and others in which the chambers 

 still persist, but are strengthened by the addition of external layers, as in 

 the fossil Belemnites ; next come forms in which the shell proper disap- 

 pears, and the super-added lamina alone appear {Loligo), and, lastly, all 

 trace of a shell is lost in the adult Octopus. Besides this, another 

 series may be traced from the closely coiled shell of the Nautilus and 

 the Ammonites through fossil forms like Crioceras, Ancyloceras, Aniso- 

 ceras, etc., until we reach the perfectly straight shells of the palaaozoic 

 Baculites and Orthoceratites. In the septa of the shells a similar grada^ 

 tion may be seen, reaching from the simple ones of Nautilus and the Orthoceratites to 

 the highly complicated ones found in the Ammonites. One more shell is all that our 



Fig. 473.— Pen or 

 internal shell of 

 Loligo pallida. 



