Cephalopoda — Dihranchiata. 



15 



been observed, when injured, to vacate its shell ; neither is the Argonauts 

 shell actually external, being always enveloped, for the greater CrALLERY 

 part, by the lobe-like expansions of the shell-secreting arms 

 (Figs. 35fl!, h). 



Four species of Argonauta are known ; they all inhabit the 



Fig. 33.— Fossil booklets of 



Lithographic Stone: Solenhofen. 



Fig. 34. — Female Argonaut in act of swimming : the arrow near the funnel (s) 

 shows the direction in which the water is being forcibly expelled from the 

 brancbial chamber ; the arrow behind the shell indicates the direction in 

 which the animal is being propelled. (Reduced.) 



Fig. 35. — a, Female of Argonauta argo, Linn., when alarmed, contracted into 

 its shell. The expanded dorsal arms still cover more than half the shell. 

 b, Shell of Argonauta with animal removed. (Reduced.) 



