386 



DECAPOD A 



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

 ehiborate 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. 



Tlie different forms of the shell appear to indicate successive 

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

 (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 - 

 ['ij^l suljstance. These laminae invest the original 



sliell, which gradually (Spirulirostra, Belosepiu) 

 loses the spiral form and becomes straight, eventu- 

 ally disappearing, while the calcareous laminae 

 alone remain (Sepia). Tliese 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 Fliragiiiopliora, Sepio- 

 p/tora, and Chondrophora} 



A. Phragmopiioka. — Arms furnished with 

 Jiooks or acetabula ; shell consisting of a pilirag- 

 of Oiiijchotoi- tiiocone or chambered sac enclosed in a thin wall 

 this sp., show- n\^Q conotheca), septa pierced by a siphuncle near 



ing the hooks ^ /' r i j i. 



and clusters of the Ventral margin (in Spinda alone this cham- 



lixiug cuslnons 1,^^.^^:^ ^^^ ^^^^^^ ^|^g j^^j^ ^^ ^j^^ gj^g^x r^^^^ 



.11111 acetal)ula _ '' 



Leiow tiieiii. 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 pj'^^o-ostracnni or 

 anterior shell, wliich may be shelly or liorny, and corresponds to 

 the gladius of the Chondrophora. The rostrum consists of 

 calcareous fibres arranged perpendicidarly to the planes of the 

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

 ■* (t>payfj.6s, i)ai'titiou ; ffrjiriov, cuttle-bone ; xo'^'^/JoSj long cartilage. 



Fi(5. 245. 



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