98 ZOOLOGY. 



rocks from the earliest period. The best known of these 

 is the ammonite, so abundant in the secondary rocks. 



The form of the body is symmetrical, the two sides 

 being alike. The head is distinct, and is surrounded by 

 a circle of arms. These are usually provided with rows 

 of suckers which act like cupping glasses. The poulpe 

 has eight arms which are all alike long. The cuttle-fish 

 has ten arms. Two of them, called tentacles, are much 

 longer than the others, and are only provided with suckers 

 at the extremities. Two of the arms of the paper nautilus 

 are provided with webs which secrete the shell. The 

 pearly nautilus has numerous arms, which are destitute 

 of suckers. The arms are considered to be modified 

 expansions of the part which corresponds to the foot in 

 other molluscs. 



130. Digestion. The mouth leads into a cavity which 

 contains two horny jaws and a tongue. The jaws work 

 vertically, and bear some resemblance to the beak of a 

 parrot. The tongue is partly covered with recurved 

 siliceous spines. It has been called by Professor Huxley 

 an odontophore, or " tooth bearer." " It consists essen- 

 tially of a cartilaginous cushion, supporting as on a pulley, 

 an elastic strap, which bears a long series of transversely 

 disposed teeth. The ends of the strap are connected with 

 muscles attached to the upper and lower surface of the 

 hinder extremities of the cartilaginous cushions ; and these 

 muscles, by their alternate contractions, cause the toothed 

 strap to work backwards and forwards, over the end of 

 the pulley formed by its anterior end. The strap conse- 

 quently acts, after the fashion of a chain-saw, upon any 

 substance to which it is applied, and the resulting wear 

 and tear of its anterior teeth are made good by the 

 incessant development of new teeth in the secreting sac 

 in which the hinder end of the strap is lodged. Besides 

 the chain-saw-like motion of the strap, the odontophore may 

 be capable of a licking or scraping action as a whole." 

 (Huxley.) The cavity of the mouth is succeeded by a 

 gullet which leads to a stomach and intestine, The 



