MANUAL or THE MOLLUSCA. 



CLASS I. CEPHxiLOPODA. 



The cuttle-fishes, though excluded hy dealers from the list of shell-fish, are 

 the most remarkable, and, rightly considered, the most interesting of any ; 

 whilst their relatives, the nautili and ammonites, are unmatched for the sym- 

 metry and wondrous architecture of their pearly shells. 



The principal locomotive organs of the cephalopods, are attached to the 

 head, in the form of muscular arms or tentacles ;* in addition to which, many 

 have fins ; and all can propel themselves by the forcible expulsion of water 

 from their respiratory chamber. 



Unlike most of the moUusca, they are symmetrical animals, having their 

 right and left sides equally developed ; and their shell is usually straight, or 

 coiled in a vertical plane. The nautilus and argonaut alone (of the living 

 tribes) have external shells ; the rest are termed " naked cephalopods," be- 

 cause the shell is internal. They have powerful jaws, acting vertically, like 

 the mandibles of birds ; the tongue is large and fleshy, and part of its surface 

 is sentient, whilst the rest is armed with recurved spines ; their eyes are large, 

 and placed on the sides of the head ; their senses appear to be very acute. 

 AJl are marine ; and predatory, living on shell-tish, crabs, and fishes. 



The nervous system is more concentrated than in the other moUusca ; 

 and the brain is protected by a cartilage. The respiratory organs consist of 

 two or four plume-like gills, placed symmetrically on the sides of the body, 

 in a large branchial cavity, opening forwards on the nnde7-\ side of the head ; 

 in the middle of this opening is placed the siphon or funnel. The sexes are 

 always distinct ; but the males are much less numerous than the females, and 

 in many species, at present unknown. They are divided into two orders, the 

 names of which are derived from the number of the Ircmcldce. 



ORDER I. DiBKANCHIATA, OwCU. 



Animal swimming ; naked. Head distinct. Byes sessile, prominent. 

 Mandibles horny (PI. I., fig. 2). Arms 8 or 10, provided with suckers. 

 Body round or elongated, usually with a pair qI fins; branchlce two, fur- 



* M. Schultze compares the arms of the cephalopods' to the oral filaments of 

 myxine. 



t According to the established usage, we designate that the under or ventral side 

 of the body, on which the funnel is placed. But if the cuttle fishes are compared 

 with the nucleobranches, or the nautilus with the holostomatous gasteropods, their 

 external analogies seem to favour an opposite conclusion. 



