188 THE MECHANICAL FUNCTIONS. 



intervening cavity. The form of this cavity is exhibited in 

 Fig. c, which represents a perpendicular section of the whole 

 organ, and where the central portion or principal mass of the 

 sucker is drawn away from the circular disk, the inner mar- 

 gin of which appears like a row of teeth. It is evident that 

 by this mechanism, which combines the properties of an ac- 

 curate valve, with an extensive cavity for producing rare- 

 faction, or the tendency to a vacuum, the power of adhesion 

 is considerably augmented.* 



So great is the force with which the tentacula of the cut- 

 tle-fish adhere to bodies by means of this apparatus, that 

 while their muscular fibres continue contracted, it is easier 

 to tear away the substance of the limb, than to release it from 

 its attachments. Even in the dead animal I have found that 

 the suckers retain considerable power of adhesion to any 

 smooth surface to which they may be applied. 



Our attention must first be directed to the remarkable fa- 

 mily of Sepias, which comprehends three principal genera, 

 namely the Octopus, the Loligo, or Calamary, (depicted in 

 Fig. 121,) and the common Sepia, or Cuttle-fish. The first 

 of these, the Octopus, which was the animal denominated 

 Polypus by Aristotle, has eight arms of equal length, and 

 contains in its interior two very small rudimental shells, 

 formed by the inner surface of the mantle. This shell be- 

 comes much more distinct in the Loligo, where it is carti- 

 laginous, and shaped like the blade of a sword. (Fig. 123.) 

 The internal shell of the common Sepia is large and broad, 

 and composed wholly of carbonate of lime: it is well known 

 by the name of the ciittlc-Jish bone. Its structure is ex- 

 tremely curious; and deserves particular attention, as estab- 

 lishing the universality of the principles which regulate the 

 formation of shells, whether internal or external, and from 



* The description I have here given is the result of my own examination 

 of a large Octopus, which I had lately an opportunity of dissecting: and the 

 annexed ^gures 123, » a, e, c, are copied from drawings I made on that oc- 

 casion. A represents the sucker in its usual form when not in action: b 

 shows the sucking surface fully expanded: and c is a section of the whole, 

 which had become somewhat flattened by the operation of dividing it. 



