24 



COMPARATIVE ANATOMY 



CHAP. 



ORDER 2. Dibranchia. 



The shell is either internal, rudimentary, or altogether wanting. When 

 present it is endogastrically coiled. There are two ctenidia, two auricles, and two 

 kidneys. The mouth is surrounded by eight or ten sucker-bearing prehensile arms. 

 The free edges of the two lobes which form the siphon have grown together. The 

 eyes are vesicular. An ink-bag is present. 



Sub-Order 1. Decapoda. 



Shell internal and often rudimentary. There are ten arms, the fourth pair being 

 developed into long prehensile tentacles, which can be withdrawn into special cephalic 

 cavities. The Decapoda are good swimmers ; their bodies are elongated dorso- 

 ventrally, and provided with lateral fins. The oviduct is unpaired. Fam. Spirulidse : 

 internal shell spirally (endogastrically) coiled. Spirula (Fig. 33). Fam. Belem- 

 nitidse : fossil forms with internal chambered shell, usually long and straight 

 (Belemnites, Spirulirostra, Belemnoteuihis). Fam. Oigopsidse ( Ommastrephes, radula 

 3.1.3, Loligopsis, Crancliia, Chiroteuthis, Owenia, ThysanoteutMs, Onychoteuthis, 

 Ommatostrephes). Fam. Myopsidse (Rossia, Sepiola, Sepiadarium, Idiosepion, Loligo 

 [Fig. 34], SepioteutMs, Beloscpia [fossil], Sepia, radula 3.1.3). 



Sub-Order 2. Octopoda. 



Without shell or "guard " (rostrum) ; eight arms ; without specialised prehensile 

 tentacles. Body thick, generally without fins, and little adapted for swimming. 



FIG. 35. Female Argonauta, in the swimming position, right aspect (after Lacaze-Duthiers). 

 1, Uncovered part of the shell ; 2, the right arm of the first (anterior) pair, with its lobe-like expan- 

 sion (sail) 3, covering a large part of the shell ; 4, fourth arm ; 5, third arm ; 6, siphon ; 7, eye ; 

 8, jaw; 9, second arm. The second, third, and fourth arms are stretched backwards inside the 

 shell. 



Oviducts paired. Fam. Cirrhoteuthidse : with fins. Fam. Philonexidse : Argonauta 

 (Figs. 35, 36, and 200, p. 243). Female with external unchambered shell. Philonexis, 

 Tremoctopus. Fam. Octopodidse (Octopus, radula 1.3.1. [Fig. 37], Eledone), 



