MOLLUSCA : CEPHALOPODA. 351 



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 rudimentary ; 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 Argortautida, 

 and the Odopodida, In the former of these there is only the 

 single genus Argonauta, (the Paper Sailor, or the Paper Nau- 

 tilus), of which the female and male differ greatly from one 

 another. The female Argonaut (fig. 146) is protected by a thin 

 single- chambered shell, in form symmetrical and involuted, which 

 is secreted by the webbed extremities 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 (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 Octopodidtz (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 ultimately 

 becomes " hectocotylised." 



SECTION B. DECAPODA. The Cephalopods of this section 

 have eight arms and two additional " tentacles," which are 

 much longer than the true arms, are retractile, and have ex- 

 panded, club-shaped extremities (fig. 143). The suckers are 

 pedunculated ; the body is always provided with lateral fins, 

 and the shell is always internal. 



This section comprises the three living families of the 

 TenthidcE, 'Sepiadce, and the Spirulida, and the extinct family 

 of the Belemnitidcz. 



The family of the Teuthidce. comprises the Calamaries or 

 Squids, characterised by the possession of an elongated body 

 with lateral fins. The shell is internal and horny, consisting 

 of a median shaft and of two lateral wings; it is termed the 

 " gladius " or " pen," and in old specimens several may be 

 found lodged in the mantle, one behind the other. In the 

 common Calamary (Loligo] the fourth left arm of the male 

 is metamorphosed towards its extremity to subserve repro- 

 duction. 



