MOLLUtsCA CEPHALOPODA. 263 



internal skeleton ; but it certainly has no pre- 

 tensions to such a designation ; for, although en- 

 veloped by the mantle, it is still formed by that 

 organ ; and the material of which it is composed 

 is still carbonate of lime. On both these ac- 

 counts it must be considered as a true shell, and 

 classed among the productions of the integu- 

 ments. It differs, indeed, altogether from bony 

 structures, which are composed of a different 

 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 fins, which are organs, de- 

 veloped in fishes, and which are supported by 

 slender bones, called raijs; but no structure of 

 this kind exists in the fins of the Cephalopoda. 



In swimming, the organs principally employed 

 by cuttle-fish for giving an effective imptdse 



* Some anaiogies have, indeed, been attempted to be traced 

 between the cartilaginous lamina 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 cepha- 

 lopoda, which are considered by some naturalists as indicating 

 an approach to the formation of an internal skeleton, analogous 

 to that of vertebrated animals. 



