ZOOLOGY. 



In the cuttle-fishes the internal shell is calcareous, and 

 lies loosely within the mantle. It consists of a broad 

 plate, which terminates behind in an imperfectly cham- 

 bered apex. It is popularly called the " cuttle-bone." 

 The squids or Calamaries (Lat. calamus, a pen) have a 

 horny internal shell, which consists of a central shaft, 

 with two lateral wings. This is termed the " pen." In 

 the SpirulsB the internal shell is whorled, disc-shaped, 

 and chambered, resembling the external shell of the 

 nautilus. The chambers are connected by a tube called 

 the siphuncle (Lat. siphunculus, a little tube). The 

 whorls do not touch one another. 



FIG. 39. A t CDTTLE-FISH (VENTRAL SIDE) ; B, " CUTTLE-BONE." 



The extinct Belemnites (Gr. belemnon, a dart) had an 

 internal shell, consisting of a phragmacoae (Gr. phragma* 

 a division; konos, a cone), a conical chambered shell, 

 which was lodged in a fibrous sheath, called "the guard." 

 This " guard" is the part usually found in the rocks, pre- 

 served as a fossil. 



The only living cephalopods, provided with external 

 shells, are the pearly nautilus (Nautilus PonypiliwJ, and 



