THE CEPHALOPODA. 169 



The branchiae are lodged in a chamber formed by the mantle ; they are symmetrical 

 in size, form, and position. In the Acetabuhfera there are two branchiae, in the Tentacu- 

 lifera four branchias ; hence the former are called Dibranchiate, the latter Tetrabranchiate 

 Cephalopods. The systemic circulation in all is aided by a muscular ventricular heart. 



The brain and nervous system are more highly developed and the ganglia more con- 

 centrated than in other Mollusca, and they have special organs for all the senses, and the 

 brain is protected in the naked species by a cephalic cartilage. 



The sexual organs are separate and exist in distinct individuals, the males are much 

 less numerous than the females, and in many species are at present unknown. 



All Cephalopods are aquatic and marine, and natation is retrograde by the forcible 

 ejection of sea water from the branchial chamber through the funnel. 



The shell is internal or external ; when internal it consists of one or more horny or 

 calcareous osselets embedded in the layers of the mantle, which serve for the protection of 

 the brain and other internal organs. The external shells are univalve, and are either 

 unilocular, as in the Argonaut (fig. 16) or multilocular, as in the Pearly Nautilus (fig. 24), 

 in which the air-chambers form so many floats for lightening the specific gravity of the 

 animal. The last chamber alone is the dwelling chamber, to which the body of the animal 

 is attached by two powerful adductor muscles and a membranous tube, which passes back- 

 wards from the body of the Nautilus to traverse all air-chambers of the shell (fig. 24). 



The Cephalopoda admit, therefore, of a natural division into two orders, characterised 

 by organic characters which do not admit of a doubt. 



In one order the head is surrounded by numerous retractile tentacula ; they have 

 four branchiae, and the Mollusc lives in the last chamber of a polythalamous shell ; 

 these form the order Tetrabranchiata or Tentaculifera. The second order have a 

 naked body (with two exceptions, the Arffoiiaufa and Spirula) : the arms are solid, fleshy, 

 flexible, and acetabuliferous, and the respiratory chamber contains two branchiae ; these 

 form the order Dibranchiata of Owen, or the Acetabulifera of d'Orbigny. All the 

 Naked Cephalopods have the power of secreting a dark-coloured fluid, very miscible in 

 water, and contained in an ink-bag formed of a tough fibrous membrane covered with a 

 thin silvery outer lamina ; this bag discharges its contents, at the will of the animal, 

 through a duct which opens near the base of the funnel. The ink, as it is called, was 

 formerly used for writing (Cicero) and in the preparation of sepia, and from its inde- 

 structible nature is often found in a fossil state; and so well is the colour of the 

 contents of the fossil ink-bags preserved in some fossils from the Upper Lias that I have 

 frequently coloured diagrams with fossil sepia collected from these Lias beds.' 



1 In the Museum of the Geological Society of London is a print of an Ichthyosaurus, drawn by De la 

 Beche with fossil sepia on a lithographic stone of English Lias. 



