386 



DECAPODA 



f 



I 



on the funnel, or vice versd. The ' resisting apparatus ' is most 

 elaborate in the pelagic genera, and least so in the more sluggish 

 littoral forms. A similar, but not so complex, arrangement 

 occurs also in the Octopoda. 



The different forms of the shell appear to indicate successive 

 stages in a regular course of development. We have in Spirula 

 (Fig. 247) a chambered shell of the Tetrabranchiate type, but of 

 considerably diminished size, which has ceased to 

 contain the animal in its last chamber, and has 

 become almost entirely enveloped in reflected folds 

 of the mantle. These folds gradually concresce to 

 form a definite shell-sac, by the walls of which are 

 secreted additional laminae of calcareous shell- 

 substance. These laminae invest the original 

 shell, which gradually (^Spirulirostra, Belosepia) 

 loses the spiral form and becomes straight, even- 

 tually disappearing, while the calcareous laminae 

 alone remain (^Sepid). These in their turn dis- 

 appear, leaving only the plate or 'pen 'upon which 

 ^ they were deposited QLoligo)^ which itself also, 

 with the shell-sac, finally disappears, surviving 

 only in the early stages of Octopus (Lankester). 



The Decapoda are divided, according to the 

 character of the shell, into Phragmophora, Sepio- 

 phora, and Chondrophora^ 



A. Phragmophora. — Arms furnished with 



hooks or acetabula; shell consisting of a phraq- 

 Fig. 245. -'Club' r. x. a ^ a ' *.\.- ii 



of Onychoteur. mocone ov chambered sac enclosed in a thm wall 



this sp., show- r^Y^Q conotJieca). septa pierced by a siphuncle near 



ing the hooks ■^, , , • /-• cr • 7 i ^i.- "U 



and clusters of the ventral margin (in iSpiruia alone this cnam- 

 fixing cushions ^^red sac forms the whole of the shell). The 

 below them, apex of the cone lies towards the posterior end of 

 ^ ^' the body, and is usually enveloped in a calcareous 



guard or rostrum. Beyond the anterior end of the rostrum the I 

 conotheca is extended forward dorsally by a pro-ostracum or| 

 anterior shell, which may be shelly or horny, and corresponds to* 

 the gladius of the Chondrophora. The rostrum consists of| 

 calcareous fibres arranged perpendicularly to the planes of the 

 laminae of growth, and radiating from an axis, the so-called 

 1 <ppayij.6s, partition ; arjTriov, cuttle-bone ; x^^^po^i ^'^^S cartilage. 



