TEGUMENTARY ORGANS— SKIN STRUCTURES. 465 



Fig. 324. — Section along the line a 6 of 

 previous figure, showing structure. 



and 281, pages 401 and 402) the mantle covers the inte- 

 rior of the shell to the very edge, and forms the shell in 

 its epidermic cells, lay- 

 er by layer, each on the 

 inside of the last, and 

 extending a little be- 

 yond it, by the growth 

 of the animal. The 

 successive growths are 

 easily seen on the out- 

 side of the shell (Fig. 

 323 ; on section, Fig. 324). If a small object like a 

 coin be slipped between the mantle and the shell it will 

 soon be covered by a secretion from the mantle, and 

 finally inclosed in the thickness of the shell. 



The subdivisions of the Acephala are seen in the 

 shells. The Monomyaria have one muscular impression 



on the shell, as in 

 the oyster; the Dim- 

 yaria have two mus- 

 cular impressions, as 

 in the clam or the 

 river-mussel, etc. 



The gastropod 

 shell grows in a sim- 

 ilar way, and the 

 successive shell mar- 

 gins can generally 

 be recognized, and 

 often form conspicu- 

 ous ornaments on 

 the shell. 



Cephalopods live, as already said, in the larger outer 

 chamber, all the others being empty and connected with 

 one another in the living animal by the siphuncle (Fig. 



Fig. 325. — Section through a nautilus shell : 

 ck, living chamber ; ch\ empty chamber : 

 si si, siphuncle ; s s, septa. A new septum 

 to be formed and an extension of the outer 

 chamber is shown by the dotted lines. 



