328 



MANUAL OF ZOOLOGY. 



being excessively tenacious in their hold, and being sometimes 

 provided with a sharp claw in the centre of each sucker. They 

 are mostly nocturnal oi crepuscular animals, and they some- 

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

 sections, Octopoda and Decaf oda, according as they have 

 simply eight arms, or eight arms and two additional "ten- 

 tacles.'' 



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. The body 

 is short and bursiform, 

 and ordinarily without 

 fins. 



This section comprises 

 the two families of the 

 ArgonautidcB, and the Oc- 

 topodidcB. In the former 

 of these there is otily the 

 single genus Argonaiita 

 (the Paper Sailor, or the 

 Paper Nautilus), of which 

 the female and male differ 

 greatly from one another. 

 The female Argonaut (fig. 

 II 7 ) is protected by a thin 

 single- chambered shell, in 

 ■ p.ipcr Nau- 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 Octopodidos (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 



Fig. 117. — Argonaiito argo, 

 tilus," female. The animal is represeuled in 

 its shell, butj the webbed dorsal arms are .se- 

 parated from the shell> which they ordinarily 

 embrace. 



