THE PHYLUM MOLLUSCA 



259 



ing of the shell remains above the mud and is used for water circula- 

 tion. Gills are absent; the mantle lining is sufficient for respiration. 

 Strings of tooth shells, which are two or three inches long, were formerly 

 used by west coast Indians as money. 



113. Class Cephalopoda: General Features 



Cephalopods are active, fast-moving molluscs. The chambered 

 nautilus (Fig. 14.13) is the most primitive of living species, with relatives 

 that are abundant as fossils dating all the way back to the beginning of 

 the known fossil record. The nautilus floats by secreting gas (resembling 

 air, but with less oxygen) into its shell. The shell is chambered, and the 

 animal lives only in the most recently added chamber. A stalk, which 

 secretes the gas, extends back through the other chambers. The shell 

 covers the animal dorsally, and is secreted by a mantle as in other mol- 

 luscs. 



The nautilus has modified the foot for both feeding and locomo- 

 tion. The anterior part grows forward on each side of the head in a 

 series of lateral lobes, at the edges of which are numerous tentacles. 

 The tentacles, annularly ridged, are able to grasp objects tightly. With 

 these the nautilus may attach to rocks while resting or may grasp prey 

 and carry it to the mouth. The posterior part of the foot is folded 

 longitudinally to form a large funnel. The posterior end fits against the 

 opening into the mantle cavity, while the smaller anterior end is sup- 

 plied with a flaplike valve. ^Vhen the funnel enlarges, water enters be- 

 tween mantle and foot, but when the funnel constricts, the posterior edge 

 of the foot closes against the mantle and the water is squirted out the 

 anterior end. By this form of jet propulsion the nautilus is able to 

 swim. Two pairs of gills lie in the mantle cavity where they are con- 

 tinually flushed with water. 



A stout pair of horny jaws assist the radula in tearing prey to bits. 



Tenlacle-S 



Funnel 



Mantle, cavity 



Figure 14.13. Lateral view of Nautilus. A diagrammatic section of the chambered 

 shell is sliown. The mantle of the left side is cut away to show the mantle cavity and two 

 of the gills. When the animal retracts, the leathery hood protects the shell opening. Com- 

 bined from several sources. 



