MOLLUSCA. 



C67 



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

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

 Palhobranchiafa. In many respects, the Conchifera are intermediate between the othei- two classes; 

 beinp; connected with the Brachiopoda through the genera Placitna and Anomia, and with the Tanicata 

 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 ail 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- 

 lusts. 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. 



OitDEE I.^DiBEANCHiATA. In tliis ordcr, 

 only one genus, Argonauta, has been liithcrto 

 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 hj a siphon or by musculiir 

 attachment. All the other genera of this 

 order are naked ; hut they are provided either 

 with an internal chambered and siphonifer- 



FlG. 1.- 



OhGANS of ClRCUr.ATION AND RESPIRATION IN CUTTLR-FISlH; 

 /eiitricli: of heart, propelling the Ijluoil by the sysR-oiic e 



bniiichcs b, 8cc, i the blood returns from the system by re, the venn cava, which (U- 



videH into two bTi\nchcs, ab. to enter the gilJK ; on these vcaselH are seated, at the h.ise 



oftlie gills, the musculiir and contractile dilatntions, cb, ivliiuh serve tlie purpose of o US shell, as in Spil-ula and BelemnOSCpia, nr 



accessory hearts, to prop(?l the blood tlirough the gills, ir, ir'. The blood returning 



from the gills is conveyed back to the ceotrrilheart by the branchial veinf;, vb. the remains of a shell are foUnd in vari0U3 



Stages of degradation, lodged in the substance of the dorsal jjart of the mantle. The armsof the Pibranchiata 

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

 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 (/iiis never exceed two 

 in number (Fig. 1, hr, hr^,) ; 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. Tbere 

 is an organ, theink-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- 

 fessor Owen Into the following families, which arc arranged under two tribes, the Octopoda, or cigbt-armed, 



