MOLLUSCA. 



211 



I.) Tetrabranchs. This order has four gills, forty or 

 more short tentacles, and an external chambered shell. 

 The partitions of the shell are united by a tube, called a 

 siphuncle, and the animal lives in the last and largest 

 chamber. These chambered shells were once very abun- 

 dant. More than two thousand fossil species are known, 

 among which are the Nautilus, Ammonite, and Ortho- 

 ceros. They have but one living representative the 

 Pearly Nautilus. This straggler of a mighty race dwells 

 at the bottom of the Indian Ocean. The shell is well 

 known, but only two or three specimens of the animal 

 have been obtained. 



2.) Dibranchs. Those having two gills. They are the 

 most active of Mollusks, and the tyrants of the lower 



FIG. 116. The Paper Nautilus, (Argonauta Argo.) Fig. i. Swimming toward the 

 point a. 2. Walking on the bottom. 3. Contracted within its shell, which is partly 

 embraced by the arms. 



tribes. There are Cuttle-fish and Poulps (or Devil-fish) 

 so large as even to be dangerous to a man who might be 

 swimming near them, and the stories of novelists like 



