LI 2 



NATURAL HISTORY. 



|.'„ ; [05 _A gill of the cuttle-fish, to show the 

 blood-vessela and the method of arrangement 

 of the u ill-leaves. 



they belong more properly to the more technical works on science. These 

 gills however, tnay be mentioned. Each consists of two tubes, one of 



which conveys the blood to, the other 

 from, the respiratory portions, which 

 consist of delicate leaf-like plates, ex- 

 tending from one tube to the other. 



Of the forms with four gills but a 

 single genus now exists, — the genus 

 In geological time allied forms were common, even abundant. 

 The older strata are filled with their shells, which figure in geological text- 

 books as Ammonites, Orthoceratites, and the like, some being of moderate 

 size, others perfectly enormous. Some of the Ammonites have a coiled 

 shell two or three feet across, while the straight Orthoceratites were 

 occasionally six or seven feet long. One of these forms, found by Pro- 

 fessor Newberry in the rocks of Ohio, weighed several tons. 



The shells of the pearly nautilus are very common to-day ; but the ani- 

 mals arc still among the rarities of museums. Why this is so does not 

 readily appear; for in the South Seas they are said to be common in water 

 of moderate depth, crawling slowly over the bottom, or swimming like a 

 squid by means of water expelled from the siphon. They are carnivorous 

 in their habits, and are often caught in fish-pots and crab-traps. Possibly 

 the fact that the natives of the South Seas are fond of them as food 

 explains this rarity. There are several points of interest concerning their 

 structure which need to be mentioned. 



First is the peculiar shell. On the outside this is dull and opaque, 



but the lining layers are as pearly as 

 those of the pearl-oyster; and sailors 

 take advantage of this structure to 

 manufacture cameos from them. They 

 paint the design on the surface with 

 grease or wax and then immerse the 

 shell in acid, which eats off all the sur- 

 face not protected, leaving the design 

 raised from a pearly ground. Even 

 more interesting is the chambered 

 structure of the shell, a feature found 

 in the fossil as well as the recent forms. 

 The animal lives in but a portion of the 

 shell, and the rest is divided by transj 

 verse part it ions into a series of chambers, all connected together by means 

 oi a tube. No perfectly satisfactory explanation of this structure has yet 



I'm. 106. *■ A section 'if tin- shell of the pearly nau- 

 tilus {Navtilus pompilius), showing at A the 

 chamber occupied by the animal, and at, a the 

 old chambers connected by a tuhe or siphunele. 



