CEPHALOPODA. 319 



branous pellicle, thrown off by the mantle when the animal was in 

 the act of advancing forwards to enlarge its shell and form a new 

 septum. 



The internal cartilaginous skeleton of the Nautilus is confined 

 to the inferior surface of the head : no part of it extends above 

 the oesophagus. Viewed sideways, it presents a triangular form ; a 

 portion of the annular brain is protected by a groove on the upper 

 surface of the cartilage : two strong processes are continued from its 

 anterior and superior angles into the crura of the infundibulum, 

 giving origin to the chief muscles of that part. Two other thinner 

 processes are continued backwards, and curve inwards and down- 

 wards : they give origin to the two great muscles which pass from 

 the internal to the external skeleton, or, in other words, attach the 

 animal to the shell. 



The muscular fibres of the head or oral sheath arise from the 

 whole of the anterior or outer part of the internal skeleton. The 

 muscular structure of the funnel presents a much greater develop- 

 ment than in the naked Cephalopods ; and, from its relation to those 

 masses which, on the one hand, attach the soft parts to the shell, and, 

 on the other, connect the head to the body, we may conclude that 

 the funnel is the principal organ of natation^ and that the Nautilus is 

 propelled, like the Octopus, by a succession of jerks occasioned by 

 the reaction of the respiratory currents upon the surrounding water. 

 The orifice of the funnel is guarded by a valve {fig. 129. i^). 



The retraction of the tentacula is done by longitudinal fibres, the 

 elongation by transverse ones. These are not, however, disposed in 

 circular or spiral series, so as to attenuate and lengthen the tentacle 

 by a general compression, but present a more complex and beautiful 

 disposition by w^hich they diminish the transverse diameter without 

 compressing the central nerve. The transverse fibres {fig. 1^2. a) 

 arise in numerous and distinct fasciculi from the dense cellular tissue 

 (fig. 132. b), surrounding the nerve in the centre of the tentacle 

 (fig. 132. d), and radiate at equal distances to the circumference; they 

 divide and subdivide as they diverge, and also send off lateral fibres, 

 which form a delicate network in the interspaces of the rays, especially 

 at the angles : the meshes include the longitudinal fasciculi, the cut 

 ends of which are shown at c, fy. 132. 



The mechanical arrangement of the contractile fibres is very similar 

 to that of the complex muscles described by Cuvier in the proboscis 

 of the elephant. The attenuation and elongation of this brobdigna- 

 gian tentacle must be effected without compressing the central 

 breathing tubes, and transverse fibres accordingly radiate from the 

 dense ligamentous tissue which surrounds them : the same prospective 



