♦ . MOLLUSC A CEPHALOPODA. 189 



which structures dlfTering much in their outward appearance 

 may result. It is composed of an immense number of tliln 

 calcareous plates, arranged parallel to one another and con- 

 nected by thousands of minute hollow pillars of the same 

 calcareous material, passing perpendicularly between the ad- 

 jacent surfaces. This shell is not adherent to any internal 

 part of the animal 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 relation. It, no doubt, is of use in giving me- 

 chanical support to the soft substance of the body, and espe- 

 cially to the surrounding muscular flesh; and thus probably 

 contributes to the high energy which the animal displays 

 in all its movements. It has been regarded as an internal 

 skeleton; but it certainly has no pretensions to such a desig- 

 nation; for, although enveloped by the mantle, it is still 

 formed by that organ; and the material of which it is com- 

 posed is still carbonate of lime. On both these accounts it 

 must be considered as a true shell, and classed among the 

 productions of the integuments. It differs, indeed, alto- 

 gether from bony structures, which are composed of a dif- 

 ferent kind of material, and formed on principles of growth 

 totally dissimilar.* 



Besides tentacula, the Sepia is also furnished with a pair 

 of fleshy fins, extending along the two sides of the body. 

 The Loligo has similar organs of a smaller size, and situated 

 only at the extremity of the body which is opposite to the 

 head. They have been regarded as the rudiments of true 

 fms, which are organs, developed in fishes, and which are 

 supported by slender bones, called rays; but no structure of 

 this kind exists in the fins of the Cephalopoda. 



In swimming, the organs principally employed by cuttle- 



* Some analogies have, indeed, been attempted to be traced between the 

 cartilag-lnous himina of the Loligo, and the spinal column of the lowest order 

 of cartilaginous fishes: these I shall have occasion to point out in the sequel. 

 Solid cartilaginous structures also exist in the interior of the body of the ce- 

 phalopoda, which are considered by some natiu-idists as indicating an approach 

 to the formation of an internal skeleton, analogous to that of vcrtcbratcd ani- 

 mals. 



