I -III I I . II 



order 1< in;iin1:iin :in eqiiili In 'nun. In effect, Hit- S-|ii:i :ml the 

 Spirilla animal- of nir.ssjvr pn>| .. >rl inn-, have need of this aid in 



swimming: and ii is m ore plentiful Ij supplied to the round -bodied 



Spirilla, than to tli<- < 'oiiolciit his. for example, tin- form of which 

 ilriiutcs .-in :iniin:il infinitely limn- auih'. In the lelenmite> the 

 :erial chambers douhtlos compensated the enormous \vei_irh1 ol 



the calcareous rostrum, which wi.uld nthci-wi<c h.-ivr rMmpclh-d 



the :ini:n:il \<> in:iin1:!in :i \crtic;il jmsil.ioii in Hie writer, r prc- 



vcntcd liori/ont.-il iiio\ cnicnt . except :it great disadvantage ; 



stl'enutli. (In the cliMMilicrcil exteni:il shells of t lie t et I'M 1 r:i D- 

 ehiiite-. represente(| .-nnoii^st theextinet ^enerii ly t he spi rnll v- 



cnileil A ninioiiites. ;m<l other ueiiern. :in<l largely developed in 



Species, \>\i\ !' which the Nimtilus i> the sole recent ex:nnple. t he 

 :iir-ch;nnliers ni;iy j)ossil)ly compensate the weii'hl of siijierin- 

 cninbent walei'. :ind l;icilit:ite its emwlinn iimveinents. if. as Lfl 

 now Ljviierjilly siijtposed, the N.-iniiliis is not ;i swiinininii' :inini:il. 

 :ind does not vnliintnrily lenve its ocenii bed. The iinineiise si/e 

 :ind weight ol' the Nautilus shell, capable ol' cont ahiin^ the entire 

 animal within its hot chamber, the absence ol' loni- arms, or web 

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



:;. Owinii' to their narrow posterior and massive anterior form. 

 as well MS to the normal direction of the siphon and the tVe|iicnt 



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



to progress throuo-h the water more rapidly in retrograde than in 

 forward motion: and this swimming is a succession of dails 

 made with m'eat velocity. Here the calcareous rostrum, as in 

 the Sepia, and which is SO largely developed in IJelemnites and 

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

 ceiving-Mini withstanding the shocks of accidental collisions. It 

 is only amonii" t he swimming species that this protection is needed, 

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

 those which inhabit the vicinity >f the coasts, like the Sepia. 



Internal shells, having no aerial chambers, show no ////////x. 

 and do not clunuv their forms at ditfeienl periods of their 

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



The lii;litiir-s <>|' tin- shell <i|' the S(>]>i;i is partly due to a. contained 

 .\hich DI-. Paul I5ert has succeeded in olitaininu in small i|iiant it ies, 

 liy o])-iiiiiL; the -.ark of the animal under \\ater. 



