256 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Water is admitted into the mantle cavity through the wide 

 cleft between the free anterior edge of the mantle and the wall 

 of the visceral sac. It bathes the gills and into it are thus dis- 

 charged the effete products of respiration. It likewise receives 

 the waste products discharged from the intestine and the prod- 

 ucts of the excretory and reproductive organs. After the ad- 

 mission of the water, the mantle cleft closes almost completely 

 by appression of its walls, and contraction forces the water out 

 through the funnel. The successive jets of water thus vio- 

 lently expelled through the funnel act as an oar in pushing the 

 animal backward ; this is its normal mode of locomotion. 



Respiration is effected through the gills. These are two pairs 

 of plume-shaped bodies attached to the wall of the mantle cavity 

 and consist of delicate lamellae with thin walls. The blood is 

 carried throughout the gills in a system of minute branches and 

 through their thin walls exchanges the waste gases it has col- 

 lected from the body for the oxygen of the water which bathes 

 the outside of the gills in the mantle cavity. It is then gathered 

 up into vessels leading back to the auricle of the heart. 



The digestive canal passes back in a straight line from the 

 mouth, through the buccal cavity, oesophagus and crop to the 

 stomach at the posterior extremity of the visceral mass. From 

 the stomach the intestine bends abruptly, develops a rounded 

 caecum and proceeds in a nearly straight line to the anus, which 

 opens into the mantle cavity. A very large digestive gland, 

 the " liver, " opens into the caecum. 



The food is largely small crustaceans, especially one species 

 of decapod ; the shells of these are easily crushed by the strong 

 jaws. These and the small fishes, and moUusks which serve 

 likewise for food, are seized by the tentacles and passed into the 

 mouth, where they are torn by the powerful movement of the 

 jaws. These jaws form a sort of beak situated just inside the 

 thick-walled buccal cavity. They are mainly composed of hard 

 black chitin but are calcified on the biting edges. So sharp are 

 the iaws and so powerful their closing muscles that Nautilus is 



