INTERNAL SHELL. 



projections upon the hitter which enter between the tubercles of 

 the opposed mantle. The complexity of this attachment increases, 

 it will be perceived, with the activity of the animal, and in the 

 comparatively sluggish littoral genera it is not found at all. 



Internal Shell. 



This is found only in the dibranchiate cephalopoda, and 

 amongst them only fully developed in the decapod division, i. e., 

 among the pelagic species, whose quick movements require the 

 support to the body which the shell affords ; nevertheless, it is 

 prefigured in the cartilaginous blades lodged in the back of the 

 Octopods, and is more fully developed in the single Octopod 

 genus, Cirroteuthis, which possesses the means of more rapid 

 motion in its large web or umbrella. 



The internal shell is simple, usually, in form, being a blade or 

 "pen lodged in a pouch or slit in the back of the mantle, with an 

 anterior, more or less specialized, prolongation of its rhachis or 

 quill. The internal shell is either horny or chitinous and trans- 

 parent, as in Loligo, or a spongy, chalky mass, as in Sepia, or 

 calcareous, as in the fossil Belemnites, or mother-of-pearl, as in 

 Spirilla ; in the latter, only, taking the spiral form of the external 

 shells of the tetrabranchiate cephalopods. 



The feather-shaped horny shell of the Loligo resembles, and is 

 called, a pen, and its rhachis, prolonged in front like a quill, com- 

 pletes the resemblance. This rhachis is on the ventral side, when 

 the pen is lodged in the mantle. The wings, or lateral projec- 

 tions, are, commonly, broader in the female than in the male 

 individuals. 



When the shell is both corneous and testaceous, as in Sepia, 

 among recent genera and in several fossil forms, it may consist 

 of a thin, horny blade, occupying the entire dorsal surface of the 

 mantle, with an underlying spongy calcareous mass attached. In 

 Sepia we find, always, the so-called aerial chambers obliquely 

 placed and not connected by a siphon, and sometimes terminated 

 by a sharp rostrum, whilst in fossil genera, as Beloptera, these 

 chambers are arranged in a single line, or in Spirulirostra they 

 become a spiral series, connected by a siphon and analogous to 

 the shell of the Spirula ;_which latter is free in the mantle, without 



