614 LECTURE XXIV. 



The primitive fibres of the muscles of the Cephalopods are smooth : 

 they are more parallel than in the lower mollusks, and are bound 

 together, so as to form well-defined flat fasciculi. 



The pallial sac is chiefly composed of a layer of cii'cular fibres, 

 interrupted behind in Sepia, where the shell is lodged. A pair of 

 round longitudinal muscles pass from the ventral wall to the base of 

 the funnel : a second pair extend from the dorsal wall to the cephalic 

 cartilage and the back part of the base of the arms. Other smaller 

 fasciculi arise from the sides of the cephalic cartilage, and are in- 

 serted into the funnel. 



Fasciculi of muscular fibres are continued from the ventral pair of 

 feet and the back part of the cranium to the muscular partition which 

 divides longitudinally the branchial cavity : other fibres descend to 

 join the muscular tunic enveloping the liver and oesophagus. In the 

 cuttle-fishes and calamaries the branchial septum is not developed. 

 The respiratory tube or funnel is a complete muscular cylinder, 

 formed by an external longitudinal, and an internal transverse, layer 

 of fibres, with which are blended the insertions of the accessoiy 

 retractor nauscles. 



The homologues of the great muscles which attach the Nautilus to 

 its shell may be traced in the different genera of Dibranchiates, 

 diminishing in size as the internal shell becomes more and more 

 rudimentary. They arise in conjunction with the fibres of the fleshy 

 tunic of the liver from the posterior part of the cephalic cartilage ; 

 but, soon quitting these fibres, they extend downwards and outwards, 

 being perforated in their course by the great lateral nerve, and are 

 inserted into the epidermic capsule of the internal shell. 



The thin shell of the female Argonauta, which is external in regard 

 to the true mantle, but internal in relation to the branchial mem- 

 branes which formed it, is retained in its position chiefly by these 

 membranes ; and when they are, as they are capable of being, 

 retracted into the cavity of the shell, it adheres to the surface of the 

 body by the adhesion of contact only and its own elasticity, not by 

 its attachment to any muscular fibres : hence the animal commonly 

 drops out of the shell when dead. 



The principal muscular fibres of the pallial fins extend, transversely to 

 the axis of the body, to the margins of the fins: they pi-esent the same 

 direction in the fossilised fins of the animal of the Belemnite. The 

 action of the powerful muscles in the terminal fins of the calamaries 

 must be aided in its effect upon the body by the elasticity of the in- 

 ternal pen or gladius. By these means they are enabled not only to 

 propel themselves forward in the sea, but they can strike the surface 

 of the water with such force as to raise themselves above it, and dart 



