60 CARTILAGES. 



that the entire structure and affinities of molluscous animals can 

 be predicated from the examination of an internal mould or a 

 morsel of shell, an};- more than that the form and habits of an 

 extinct quadruped can be inferred from a solitary tooth or the 

 fragment of a bone.* 



" Who has not admired the beauty of shells ? " exclaims 

 Carpenter,'!' " the rich lustre of the Cowries ; the glossy polish of 

 the Olives ; the brilliant painting of the Cones ; the varied layers of 

 the Cameos ; the exquisite nacre of Mother-of-pearl ? Who has not 

 listened to the mysterious ' sound of the sea ' in the Whelks and 

 Helmets, or wondered at the many chambers of the Nautilus? 

 What child ever went to the sea-shore without picking up shells; 

 or what lady ever spurned them as ornaments of her parlor ? 

 Shells are at once the attraction of the untutored savage, the 

 delight of the refined artist, the wonder of the philosophic 

 zoologist, and the most valued treasures of the geologist. They 

 adoi"n the sands of sea-girt isles and continents now; and they 

 form the earliest ' footprints on the sands of time ' in the history 

 of our globe." 



CARTILAGES. 



MoUusks have no internal skeleton comparable to that of the 

 Yertebrata, but in the Cephalopoda the most active and highly 

 organized of the mollusca are found cartilaginous supports for 

 some of the organs, partially replacing the vertebrate skeleton. 

 Those of the principal nerve-gangiiee are well developed, some- 

 times completely enveloping them ; besides which the principal 

 organs of sense, the valves of the mantle, the fins, etc., are duly 

 provided. 



As might be expected from its habits, the cartilaginous 

 system of the Nautilus is the most simple of all the cephalopods, 

 consisting of a well-developed head-cartilage, so shaped and 

 situated as to support the oesophageal ring, the cerebral and pedal 

 commissures, whilst two prolongations of it serve the funnel or 

 siphon. 



In the dibi'anchiata, unlike the Nautilus, the head-cartilage 

 forms a complete ring around the oesophagus ; from the medial 

 line of the back of this ring spring two lancet-form cartilaginous 

 processes, the cartilages of the ej^elid, and the under side of the 

 same ring spreads into a spoon-shaped process which comes far 

 forward and supports the eyes ; particularly when, as in Sepia, 

 it reaches to the sides of the head and encompasses the ocular 

 opening. 



* Etudes Critiques sur les Mollusques Fossiles, par L. Agassiz, NeU' 

 cJiatel, 1840. 



•j- Lectures on Mollusca, Smithsonian fieport, 1860. 



