MOLLUSCA. 



667 



former oy means of four branchial leaflets, whence they are distinguished as Lamellihranchiata ; 

 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 A?iomia, 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 



c gills, which is one of their best marked 



*"' characters; but they differ also in many 



other particulars. 



Oeder I. — DiBRANCHiATA. 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 siphouifer- 

 ous shell, as in Spirula and Delemnosepia, or 

 the remains of a shell are found in various 



Fig. 



c, systc 

 branch' 



.—Organs of Circulation and Rhspiration in Cvtti.f.-fish; 



ic ventricle of heart, propelling the bluod by the systemic artery a*, and its 



6, &c. ; the blood returns from the system by fc, the vena cava, which di- 

 nto two branches, ab. to enter the gills ; "on these vessels are seated, at the base 

 of the gills, the muscular and contractile dilatntions, cb, which serve the purpose of 

 accessory hearts, to propel the blood through the gills, bT,br\ 'l"he blood returning 

 from 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 nimiber ; but m many gciier i t^\o longer tentacles are superadded. Both kinds 

 of prehensUe 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 oihearing is distinctly deve- 

 loped. The piJJs never exceed two 

 in number (Fig. 1, hr, 6cl,) ; but 

 the branchial circulation 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 inh-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 divided by Pro- Fig. 2 Ocrnprs on rori.r. 



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



