212 IXVEKTEBRATE ANIMALS. 



the purpose of conveying the sperm-cells to the female. For the 

 above reason, the general name of " hectocotj-lus " is given to the 

 detached reproductive aim nf an)' of the Cuttle-fishes (tig. 150, b). 



Tlie s/ic/l of tiie Cqilnilnpinhi is sometimes external, sometimes 

 internal. The internal skeh^tmi is seen in most of tlie Cuttle-fishes, 

 in which it is known as the "cuttle-bone" or "jien." It maybe 

 either horny or cak"ireous, and it is sometimes complicated by the 

 additiii)! of a chandii^red portion. Tlie only living Cephalopods 

 which ai'c [iroviiled with an e.xteiaial sliell are the I'ajier Nautilus 

 (AriiitiiiniUi) ;ind tlie Pearly Nautilus (Xnutilus pniii piliiix); but not 

 only is tlie structure of the animal different in each of these, but 

 tlie nature of the shell itself is entirely diffeient. The shell of the 

 Argonaut (fig. 151) is coileil into a spiral, but it is not divided into 

 chaudiei's, and it is secreteil by the webbed exti'cnuties of two of the 

 dorsal arms of the female. These arms are bent backwards, so as to 

 allow tlje animal to live in the shell ; but there is no organic connec- 

 tion between the shell and the boily of the animal. The .shell of the 

 Pearly Nautilu.s, on the othei- liand, is secreted liy the mantle, and 

 is org.anically connected to the animal. It is coiled into a sparal 

 (tig. 155). liut it diH'ers from the sliell of the Argonaut in being 

 di\'ided into a scries of chambers by means of slielly partitions, 

 whicli are connected together by a tube or " si]jliuncle," the animal 

 itself living in the last and largest chamber only of the .shell. 



The (_':'j:liiilnjiiiili are divided into two extremely distinct and 

 natural orders, termed Tv^\tri-i\\r\y DiJirmii'JiintK and I'drahrancliUitu, 

 according as they have tira <,y /,i,ir gills or brancliia'. 



Tlie JJiliniiir/iiiifii i'oui|irise tin- ( 'uttle-fishes, Sipiids, C.alamaries, 

 and I'aper Nautilus, and they are characterised by lit-ing almost 

 iuv;iri.-ibly dt-.stitute of :iny external sliell ; by iicvir having more 

 th.an eight or ten arni.s, which are always furnished with suckers ; 

 liy having only two gills, wliicti are ]a-oviiled with "branchial 

 hearts'"; by the po.ssesjsion of an "iiik-b.ag"; .and by the f.act that 



till' "fill 1" forms a. coni]ilrtr tube. They are di\-idrd into two 



sections — 0,Y,,y;,,r/,( and /AT.^yio,/,/ - - a./cordiiig .as they li.ave only 

 eight arms (tig. ] 50), or eight arms with t«o additioii.al longer pro- 

 cpssrs or " triit.acles" (fig. ]5:i). Amongst the 0,tu/Hi,li are the 

 Paper Nautilus .and the Poiilpps (CAy„^,/,xi. The Paper Nautilus is 

 found ill the N\arnier seas of \-,arious p.arts of the world, generallv 

 lloatiiig at. the surface. 'The twd sexes dJHVr, as ali'eadv said, 

 greatly in external appearance. Hir female (fig. b5b) inhabits a 

 beautiful on(>-cli;imiiered cell, which is semMrd by the \\'ebbfd ex- 

 tivmitics of two of the dor.sal arms. The shell is n.it in any way 

 att;,rl„.l to Ihi' body of tho ;iiiiuial, but the wrl.bod .•iriii.s are turned 

 backw.ards, and the animal sits ill the shell with the funnel turned 



