386 



DECAPODA 



CHAP. 



on the funnel, or vice versa. 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 (^Loligo)^ 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- 

 pho7'a, and Chondrophora.^ 



A. Phragmophora. — Arms furnished with 

 hooks or acetabula ; shell consisting of a pJirag- 



mm 



Fig 24:5. * Club ' 



of Onychoteu- mocone ov chambered sac enclosed in a thin wall 

 this sp., show- rii^Q co7iotheca^, septa pierced by a siphuncle near 



ic the hooks -^ ' x x j i. 



and clusters of the ventral margin (in Spirula alone this cham- 



fixing cushions bered 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 



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 (ppayix6s, partition ; ariTriou, cuttle-bone ; x'^'^Spos, long cartilage. 



