Chap. III.] 



CEPHALOPODA. 



name from the possession of two pairs of gills, with 

 which, an exceptional circumstance among Molluscs, 

 are associated two pairs of ante-chambers to the ven- 

 tricle. The siphon is incomplete, the propodial ten- 

 tacles are numerous and devoid of suckers ; the shell 

 in external, chambered, and coiled (Fig. 40). 



B. The Dibraiicliiata have either eight arms as 

 in the Octopus, or ten as in the squid (Loligo), or Sepia ; 



Fig. 40.- Section of the Shell of the Pearly Nautilus, showing the coil 



of chambers, and the animal in the largest, or that last formed (z). 

 a, Mantle; b. dorsal fold; .7, shell-muscle; -ii. Siphuncle; fr, funnel or siphonal 

 tube; n, hood;p, tentacles; s, eye; x, eepta between the chambers. 



there is only a single pair of gills and auricles, and 

 the arms are provided with suckers (Fig. 41). 



It has long been the custom to divide the members 

 of the Animal Kingdom sharply into the two great 

 groups of " Vertebrata " and " Invertebrata"; we have 

 seen, however, that the most scientific separation is 

 that into uni-cellular and multi-cellular organisms, 

 Protozoa and Metazoa ; and, next, that the lower 

 Metazoa have no signs of that body-cavity or ccelom 

 which becomes so well marked a part of the organisa- 

 tion of the higher forms ; and, lastly, we have seen 

 that the Echinodermata, the Arthropoda, and the 



