MOLLUSCA : CEPHALOPODA. 



305 



are mostly nocturnal or crepuscular animals, and they some- 

 times attain to a great size. They may be divided into two sec- 

 tions, 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 suck- 

 ers. The shell is internal 

 and rudimentary ; in one 

 instance only (the Argo- 

 naut), external. 



This section comprises 

 the two families of the 

 Argonautidce, and the Oc- 

 topodidcz. 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. 

 109) is protected by a thin 

 single-chambered shell, in 

 form symmetrical and in- 

 voluted, which is secreted 

 by the webbed extremi- 

 ties 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 toward the keel, and the webbed 

 arms applied to the shell. The male Argonaut is much 

 smaller than the female (about an inch in length), and is not 

 protected by any shell. The third left arm 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 Octopodida (or Poulpes) there are eight arms, all 

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

 is an internal 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 ulti- 

 mately becomes " hectocotylised." 



VOL. i. 



Fig. 109. Argonauta argo, the "Paper Nau- 

 tilus," female. The animal is represented in 

 its shell, but the webbed dorsal arms are se- 

 parated from the shell, which they ordinarily 

 embrace. 



