348 LECTURE XXIV. 



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 accessory 

 retractor muscles. 



The analogues of the great muscles which attach the Nautilus to 

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

 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 Argonauta, whicli is external in regard to the 

 true mantle, but internal in relation to the brachial membranes 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 present 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 eff'ect 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 like 

 the flying fish for a short distance through the air. This is the 

 highest act of locomotion, the nearest approach to flight, which 

 any of the molluscous animals have presented. 



We find associated with the varied and active powers of lo- 

 comotion just described, visual organs of large size and singular com- 

 plexity of structure. The whole surface of the body is highly sen- 

 sitive, with a concomitant development of the nervous centres, which 

 exhibit the highest conditions observable in the Invertebrate series of 

 animals. 



The brain is inclosed in a cartilaginous cranium, together with a 

 portion of the oesophagus, from which it is separated by the mem- 

 brane analogous to the dura mater. Between that part of the fibrous 

 membrane which lines the cerebral cavity and the pia mater covering 

 the brain, there is an intervening space filled with a gelatinous 

 arachnoid tissue. In the cuttle-fish, the supra-oesophageal cerebral 



