OR OSSEOUS SYSTEM. 55 



cartilaginous, internal shells, from the upper edge of the 

 mantle to the point of the tail, being contained loose in 

 a closed dorsal sac of the mantle. In the octopus, the 

 dorsal lamina is wanting, but there are two small lateral 

 stiliform, loose, cartilaginous pieces contained in the sub- 

 stance of the mantle, as the median shell in other 

 species. 



In this highly complicated class of molluscous ani- 

 mals, which approach so near to the cartilaginous fishes 

 in the structure of many of their internal parts, we already 

 find several internal rudimentary pieces of an organized 

 cartilaginous skeleton. The brain encompassing the eeso- 

 phagus is enclosed in a large curved cranical .bone, which 

 forms also part of the orbits on each side, contains the 

 cavities of the ears, and has numerous muscles inserted 

 into it. Other cartilaginous, organized pieces are seen 

 extending downwards from the back part the skull, like 

 the rudiment of a vertebral column. Two clavicular 

 pieces in front unite to the first rudiments of a sternum, 

 and attach the sides of the mantle to the trunk, and 

 there are generally two scapular pieces, more or less firm, 

 extending along the sides to which the muscles of the 

 lateral fins are attached. These two scapular pieces are 

 seen in the sepiola vulgaris, (Fig. 26. 1.) b, where they 

 support the arms (c, c } ) and are freely moveable on the 

 back, like the scapulae of vertebrata. We also perceive 

 muscles inserted into the strong upper and lower man- 

 dibles (Fig. 26. 5. 0, b,) of these animals, formed like the 

 bills of a parrot. The suckers of the arms, (Fig. 26. 4. 

 b, c,) whether sessile (c,) or pedunculated (b } ) have their 

 inner circular margins supported, each by a firm cartila- 

 ginous circular plate (,) with minute sharp teeth extend- 

 ing inwards from one of its sides, by which the action of 

 these prehensile organs is aided. In the onychia, there 

 are dense, sharp, curved, conical spines placed in these 

 suckers, like the conical teeth disposed on the oral disk 

 of lampreys, among the lowest of the cartilaginous 

 fishes. 



