4 EOCENE MOLLUSCA. 



tube called the siphon or siphuncle, which perforates and traverses the chambers of all 

 the multilocular shells, whether external or internal,* and by means of which, as it has 

 generally been supposed, the animal can diminish or increase the specific gravity of the 

 shell, and so facilitate its rising or sinking in the water. 



In the recent Nautili, the sole living representatives of the tetrabranchiate Cepha- 

 lopods, the lateral hearts are wanting, the enlarged surface of the branchial apparatus 

 rendering such additional means of circulation unnecessary. 



The funnel or locomotive tube is placed beneath the head, and supports at its base 

 the apparatus for resistance before noticed. Its functions are various ; it conveys 

 away the water inhaled for respiration after that object has been served, and, as we 

 have already seen, becomes, at the will of the animal, the principal locomotive agent ; 

 it is also the excretory tube. The condition of this organ is used by Professor Owen 

 as an ordinal character ; in the dibranchiate Cephalopods the parietes of the funnel 

 are entire, while in the tetrabranchiate Cephalopods they are disconnected along the 

 ventral margins. 



A peculiar provision for defence is found among the naked Cephalopods, which is 

 denied to those protected by an external shell ; this provision consists of an organ for 

 secreting and expelling an inky fluid, by the effusion of which the animal, when 

 alarmed, is enabled to discolour the surrounding water, and thus to facilitate or conceal 

 its escape. The fluid is contained in a bladder-shaped sac, called the ink-bladder, and 

 its presence may be regarded as a certain indication of the dibranchiate type of 

 organization, f 



In addition to the retrogressive power possessed by all the Cephalopods, and 

 derived from the agency of the funnel, the decapodous genera are provided with lateral 

 or terminal fins, more or less coriaceous, according as the habits of the animal are 

 more or less pelagic or littoral. The motive function of the fins, however, appears 

 to be secondary ; those organs being used chiefly to sustain or steady the animal, and 

 direct its course through the water. The position of the fins is used as a generic character. 



The dibranchiate Cephalopods carry on their heads eight or ten arms, the place of 



* In M. de Blainville's " Memoire sur 1' Animal de la Spirula et sur 1' Usage du Siphon des Coquilles 

 Polythalames," the siphuncle is described as a solid tendinous prolongation of the retractor muscles, by 

 means of which the animal is enabled to withdraw the cephalic mass within a cavity formed by the anterior 

 extremity of the mantle, and thus to regulate the specific gravity of the body. It appears, however, from 

 Professor Owen's examination of two specimens of S. Peronii {fragilis), captured and brought home by 

 Captain Sir Edward Belcher (see Zoology of the Voyage of the Samarang), that the soft or membranous 

 siphon is in reality a tube continued from the calcareous siphon and the last chamber of the shell, through 

 a semicircular aperture in the mantle, into the visceral cavity. 



f M. d'Orbigny, after referring to these means of escape in the Sepise, says (Moll. Viv. et Foss., vol. i, 

 p. 134), that he is far from believing that the faculty is enjoyed by every species ; and that, in fact, if it 

 exists among the Sepidse, it is at the least doubtful among the other Cephalopods, who possess but a small 

 quantity of the liquid, which they only expel when dying. 



