MOLLUSCA : CEPHALOPODA. 



427 



sections, Octopoda and Decapoda, according as they have simply 

 eight arms, or eight arms and two additional " tentacles." 



SECTION A. OCTOPODA. The Cephalopods comprised in this 

 section are distinguished 

 by the possession of not 

 more than eight arms, 

 which are provided with 

 sessile suckers. The shell 

 is internal and rudiment- 

 ary ; in one instance only 

 (the Argonaut) external. 

 The body is short and 

 bursiform, and ordinarily 

 without fins. 



This section comprises 

 the two families of the 

 Argonattttdce, and the Oc- 

 topodidiz. In the former 

 of these there is only the 

 single genus Argonauta 

 (the Paper Sailor, or the 

 Paper Nautilus), of which 

 the female and male differ 

 greatly from one another. 

 The female Argonaut (fig. 

 230) is protected by a 



tViin c-*WA> /-^/rw/Ww/cVi^ll Fi S- 2 3- Argonauta argo, the "Paper Nau- 



tnm single-cnamtjerea sneii, j ilus/ , female . The animal is repr esented in 



in form Symmetrical and its shell, but the webbed dorsal arms are se- 

 i , j i i parated from the shell, which they ordinarily 



involuted, Which IS Secret- embrace. 



ed by the webbed extrem- 

 ities of the dorsal arms, but is not attached in any way to the 

 body of the animal. It sits in its shell with the funnel turned 

 towards the keel, and the webbed arms applied to the shell. 

 The male Argonaut is much smaller than the female (less than 

 an inch in length), and is not protected by any shell. The 

 third left arm of the male (fig. 228) is developed in a cyst, and 

 ultimately becomes a "hectocotylus," and is deposited by the 

 male in the pallial chamber of the female. 



In the Octopodidce. (or Poulpes) there are eight arms, all 

 similar to one another, and united at the base by a web. 

 There is an interhal rudimentary shell, represented by two 

 short styles encysted in the substance of the mantle (Owen). 

 The body is seldom provided with lateral fins. The third right 

 arm of the male is primarily developed in a cyst, and ultimately 

 becomes " hectocotylised." 



