262 THE MECHANICAL FUNCTIONS. 



two very small rudimental shells, formed by the 

 inner surface of the mantle. This shell becomes 

 much more distinct in the Loligo, where it is 

 cartilaginous, and shaped like the blade of a 

 sword. (Fig. 123). The internal shell of the 

 common Sepia is large and broad, and com- 

 posed wholly of carbonate of lime : it is well 

 known by the name of the cuttle-Jish hone. Its 

 structure is extremely curious ; and deserves 

 particular attention, as establishing the univer- 

 sality of the principles which regulate the forma- 

 tion of shells, whether internal or external, and 

 from which structures differing much in their 

 outward appearance may result. It is composed 

 of an immense number of thin calcareous plates, 

 arranged parallel to one another and connected 

 by thousands of minute hollow pillars of the 

 same calcareous material, passing perpendicu- 

 larly between the adjacent surfaces. This shell 

 is not adherent to any internal part of the ani- 

 mal which has produced it ; but is enclosed in 

 a capsule, and appears like a foreign body, 

 impacted in the midst of organs, with which 

 at first sight, it would appear to have no re- 

 lation. It, no doubt, is of use in giving me- 

 chanical support to the soft substance of the 

 body, and especially to the surrounding mus- 

 cular flesh ; and thus probably contributes to 

 the high energy which the animal displays in 

 all its movements. It has been regarded as an 



