MOLLUSCA. 



667 



former oy means 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 

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

 being connected with the Brachiopoda through the genera Placuna and Anomia, 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- 

 • br 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. 



6ri 



ab 

 ob 



ab 



e 



bu 



cp 



Fig. 1.— Organs of Circulation and Respiration in Cuttle-fish ; 

 e, systemic ventricle of heart, propelling the blood by the systemic artery as, and its 

 branches ft, &c. ; the blood returns from the system by cc, the vena cava, which di- 

 vides into two branches, 06. to enter the gills ; on these vessels are seated, at the base 

 of the gills, the muscular and contractile dilatations, rb, which serve the purpose of 

 n' cesaory hearts, to propel the blood through the gills, br,br\ The blood returning 



at I 



Order I. — Dibbanchiata. In this order, 

 only one genus, Jrgonauta, 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 

 order are naked ; but they are provided either 

 with an internal chambered and siphonifer- 

 ous shell, as in Spirilla and Eelcmnoscpia, or 

 the remains of a shell are found in various 



fr the gills is conveyed back to the central heart by the branchial veins, vb 



stages of degradation, lodged in the substance of the dorsal part of the mantle. The arms 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 acetabula, or suctorial 

 disks for adhesion. The jaws are 

 horny, and their margins tren- 

 chant. The eyes are sessile, and 

 of a structure approaching those 

 of fishes in perfection. The or- 

 gan of hearing is distinctly deve- 

 loped. The gills never exceed two 

 in Dumber (Fig. 1, br, br 1 ,) ; but 

 the branchial rirculation is aided 

 by two muscular ventricles, situ- 

 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 ia divided by Pro- Pro. 2~Oci 



fessor Owen into the following families, which are arranged under two tribes, the Octopoda, or . ilghtarmed, 



c^: 



