CLASSIFICATION 41 



way, and the shell is principally held in place and 

 protected on the exterior by the anterior pair of 

 arms, which are furnished with web-like expansions 

 for the purpose. The ends of the remaining arms, 

 which are carried folded back over the animal's body, 

 are tucked into the shell, and seemingly also assist 

 in retaining it by the aid of the suckers. It is 

 commonly stated that the shell is entirely the pro- 

 duct of the pair of webbed arms, and that once the 

 animal quits its tenement it cannot re-occupy it ; but 

 both statements are incorrect. The latter has been 

 shown by actual observation to be the reverse of 

 fact, while the occurrence of fractured shells, showing 

 evidences of repair from the inner side, upsets the 

 former statement, which is otherwise wanting in 

 probability. The more reasonable supposition is 

 that the shell is mainly secreted by the mantle cover- 

 ing the visceral dome, and that the webbed arms, 

 which are also furnished with secretory cells, con- 

 tribute to the exterior and assist in moulding it, 

 during formation, to the animal's body. 



The Cephalopoda are divided into — 



Order I. : Tetrabranchia (Plate XXII.), or those 

 having four gills and an external shell. The funnel 

 is in two parts, and the eyes are open, having no 

 crystalline lens. 



Sub-Order 1 : Nautiloidea, or the Nautilus and 

 its allies. 



Sub-Order 2 : Ammonoidea, or the extinct group 



