THE SHELL- ANIMALS 



413 



At the end of the body is a pair of broad fins used for 

 swimming slowly forward. On the lower side of the neck is 

 a siphon or funnel from which water may be ejected with 

 such great force as to drive the animal rapidly backward. 

 At the same time it may eject an inky fluid, so as to cloud 

 the water. From a similar ink of a cuttle- 

 fish India ink or sepia was originally made. 



The cavity out of which the head pro- 

 trudesjs the mantle-cavity, and the outer 

 surface or " skin " of the body is the mantle. 

 It contains pigment spots of various colors, 

 which are very changeable during life. 



Externally there is no evidence of a shell, 

 but beneath the " skin " on the upper side 

 is a quill-like structure (the pen) four to six 

 inches long. The same structure in cuttle- 

 fishes is the cuttle-bone sold for use of caged 

 birds. Comparative studies have shown 

 that the pen and cuttle-bone represent 

 shells such as that of the chambered 

 nautilus. 



Giant squids with arms forty feet long 

 and with eyes a foot in diameter have been 

 captured. 



The octopus or devil-fish has a structure similar to a squid ; 

 but its body is shorter, there are eight arms and there are no 

 fins. Some species are of enormous size, but not large enough 

 to attack a ship, as described in old books of fiction. Speci- 

 mens weighing two hundred pounds and with arms five feet 

 long have been seen. 



The chambered or pearly nautilus, made famous in the 

 poem by Oliver Wendell Holmes, is similar to an octopus or 

 squid, but develops' a many-chambered shell. It lives in 

 the outer, largest chamber, and from time to time secretes 

 partitions across the shell so as to leave behind an out- 



FIG. 146. Male 

 squid, a, arms 

 two long ones in 

 male ; e, eye ; i, 

 opening to siphon ; 

 /, fin. (After 

 Verrill.) 



