Div. 2. MOLLUSCA.— CEPHALOPODA. 



381 



former by mean:i of four branchial leaflets, whence they are distinguished as Lamellibranchiata ; 

 whilst in the latter the function is performed by means of the mantle itself, whence they are called 

 Palliobranchiata. In many respects, the Conchifera are intermediate between the other two classes ; 

 being connected with the Brachiopoda through the genera Placuna and Anumia, and with the Tunicata 

 through the order Inclusa. The Conchifera and Tunicata being thus raised to the rank of classes, 

 their primary subdivisions will be orders instead of families. 



CEPHALOPODA. 



The researches of Professor Owen upon the structure of the animal of Nautilus, and upon its rela- 

 tions to the other Cephalopoda, have led him to propose a new arrangement of this class, which is 



now generally adopted. The entire class 

 is divided into two orders ; of which the 

 first, including nearly all existing spe- 



hr cies of Cephalopods, approaches most 

 nearly to vertebrated animals ; whilst the 

 second, which contains only one existing 



"^ genus, but to which a great number of 

 fossil forms are to be referred, is more 

 closely allied to the Gasteropodous Mol- 

 lusks. These orders are named according 

 to the difference in the number of their 

 gills, which is one of their best marked 

 characters; but they differ also in many 

 ^ ,.^^ „,. ^^^-^ ,-, .^ .^. other particulars. 



oft ' /.-«/. -^^ ^ -r . ^-. .^ .^^ 



6cl 



c 



6(1 



cp 



Obdeb I. — DisBANCHiATA, In this order, 

 only one genus, Argonauta, has been hitherto 

 found, in which the body is protected by an 

 external shell ; this consists of but one cham- 

 ber, and does not adhere to the body of its 

 occupant, either by a siphon or by muscular 



attachment. All the other genera of this 



Pig. 1.— Organs of Cipculation and Respiration in Cuttle-fish-. , j v ^ i, j a -tv, 



c, systemic ventricle of heart, prorelling the blood hy the systemic artery as, and its Order are naked ; bUt they are prOViaed eitner 



brntiches b, &c. ; the blood returns from the system hy pc, the vena cava, which di- ^y\^\^ nn internal chambered and siphonifer- 



vides into two branches, afc, to enter the gills ; on these vesBels are seated, at the base ' 



of the gills, the muscular and contractile dilatations, c6, which serve the purpose of OUS shell, aS in Spirula and BelemnOSepioi, Or 



accessory hearts, to propel the blow! through the gills, br.br\ The blood returning . ^ , ,, r j *-. _• « 



from the gills is conveyed back to the central heart by the branchial veins, vb. the remamS of a sheU are fOUnd in VanOUS 



Stages of degradation, lodged in the substance of the dorsal part of the mantle. The orms of the Dibranchiata 

 are, properly speaking, eight in number ; but in many genera, two longer tentacles are superadded. Both kinds 

 of prehensile organs are provi- 

 ded with aeetdlmla, or suctorial 

 disks for adhesion. The jaws are 

 horny, and their margins tren- 

 chant. Thee^«s are sessile, and 

 of a structure approaching those 

 of fishes in perfection. The or- 

 gan of hearing is ilistinctly deve- 

 loped. The ^tfk never exceed two _ 



in number (Fig. l,6r, brl,) ; but '^j., ll^'^te^^^^H^liM^JS^^^-'*^^^^^ \^ 



tlie branchial circulation is aided \ mifj^^^^^}?^^^^^^^^^'' ^S^f^^^ iW ___ u 



by two muscular ventricles, situ- ji'^ \ J^|a ^ft^tiS^s^SSaB&l'?^^^^^®^^ B^ J^^''^^\ 



ated one at the base of each gill 

 (cb.) ; hence there are three dis- 

 tinct hearts in this order. There 

 is an organ, the ink-bag, for secre- 

 ting and expelling a black fluid, 

 used as a means of concealment. 

 The parietes of the funnel are 

 entire. 



This order is di^-ided by Pro- Fig. 2— Octi.pus or Poui.r. 



fessor Owen into the following families, which are arranged under two tribes, the Octopoda, or eight-armed, 



