272 Introdzictwn to Animal Morphology. 



CHAPTER XXXVI. 



CLASS 2. — CEPHALOPHORA [BlatflVtlle). 



Otocardians with a head, bearing sense-organs, and 

 a mouth with, on its floor, a masticatory apparatus or 

 radulay placed on an eminence partly muscular and 

 partly cartilaginous ; on this radula are transverse 

 rows (joints) of irregularly shaped, but regularly dis- 

 posed, teeth of conchiolin. Except in the first sub- 

 class, the body is not fully covered by the mantle, 

 which usually secretes a one-valved shell, often with 

 a separate posterior, but not articulating portion. 

 Three sub-classes are included : — 



Sub-class I. Scaphopoda [Bromi) — marine, dioe- 

 cious, symmetrical forms, linking the next group to 

 the bivalves ; forming whitish, small, tubular, ele- 

 phant' s-tusk-shaped shells, concave dorsally, and open 

 at both ends. 



The shell is concentrically laminated, each new layer 

 underl3dng its predecessor, and extending farther forward, 

 with no periostracum, and thicker behind than before ; its 

 surface layer is homogeneous, on a prismatic stratum whose 

 columns bend inwards and outwards alternately as in Patella ; 

 the lining vertically striated laminae appears crenate on 

 transverse section. The oldest part (narrow end) of the shell 

 is often traversed by a fungus. Two or four oblique dorsal 

 muscles (like the columellaris of Gasteropoda) attach the 

 animal to the shell. 



The mantle is thick in front, and has no outer 

 epithelium nor glands, but many pigment specks and 

 transverse ciliated folds ; posteriorly it has a spoon- 



