TNTEENAL SHELL. 11 



order to maintain an equilibrium.* In effect, 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 

 ferial chambers doubtless compensated the enormous weight of 

 the calcareous rostrum, which would otherwise have compelled 

 the animal to maintain n vertical position in the water, or pre- 

 vented horizontal movement, except at great disadvantage to its 

 strength. (In the chambered external shells of the tetrabran- 

 chiates, represented amongst the extinct genera by the spirally- 

 coiled Ammonites, and other genera, and largely developed in 

 species, but of which the Nautilus is the sole recent example, the 

 air-chambers may possibly cornpensate the weight of superin- 

 cumbent water, and facilitate its crawling movements, if, as is 

 now generally supposed, the Nautilus is not a swimming animal, 

 and does not voluntarily leave its ocean bed. The immense size 

 and weight of the Nautilus shell, capable of containing the entire 

 animal within its last chamber, the absence of long arms, or web 

 or tins, all seem to favor this supposition as to its habits.) 



3. Owing to their narrow posterior and massive anterior form, 

 as well as to the normal direction of the siphon and the frequent 

 use of the webbed arms in swimming, the cephalopoda are able 

 to progress through the water more rapidly in retrograde than in 

 forward motion; and this swimming is a succession- of darts 

 made with great velocity. Here the calcareous rostrum, as in 

 the Sepia, and which is so largely developed in Belemnites and 

 other fossil genera, comes into use as a body-protector, in re- 

 ceiving and withstanding the shocks of accidental collisions. It 

 is only among the swimming species that this protection is needed, 

 and it is most required, and consequently most developed, in 

 those which inhabit the vicinity of the coasts, like the Sepia. 



Internal shells, having no aerial chambers, show no nin-l<>n*, 

 and do not change their forms at different periods of their 

 growth ; but in those furnished with the air-chambers, a distinct 



* The lightness of the shell of the Sepia is partly due to a contained 

 gas, which Dr. Paul Bert lias succeeded in obtaining in small quantities, 

 by opening the sack of the animal under water. 



