8 7 2 



DIBRANCHIATE CEPHALOPODS. 



secreted by the two dorsal arms, and has no connection with the 

 mantle of the animal. In the great majority of the Dibranchi- 

 ates, however, there exists an internal shell which is contained within 

 the mantle, and varies in structure in different types. In some 

 cases, the shell has the form of an elongated, horny, feather-shaped 



Fig. 804. — a, The Common Calamary (Loligo vulgaris), reduced in size : a, One of the ordinary 

 arms ; t, One of the longer arms or "tentacles." B, Skeleton or "pen " of the same, one-fourth 

 natural size (after Woodward), c, Side-view of one of the suckers, showing the horny hooks 

 surrounding the margin, d, View of the head from in front, showing the bases of the arms {a) 

 and tentacles (t), the mouth (m), and the funnel (_/"). 



body or "pen" (fig. 737, b), which is situated dorsally in a closed 

 sac of the mantle. In other cases, as in Septa (fig. 737, a) the 

 internal shell is calcareous in composition. The posterior end of 

 this skeleton may exhibit in a rudimentary form a chambered sac 

 or "phragmacone," but in many cases no trace of this structure can 



