MOLLUSCS. 



211 



ORDER L--TETRABRAXCHIATA. 



In the Tetrabranchs there are two pairs of gills (i.e., four 

 in all) ; the head bears numerous short tentacles without 

 suckers, and the body is enclosed in a chambered shell. 

 The pearly nautilus is the only living representative of this 



FIG. 46. Female Nautilus, the shell laid open (from Lud wig's Leunis). 1, man- 

 tle; 2, dorsal lobes; 3, tentacles; 4, head fold; 5, eye; 6, siphon; 8, shell 

 muscle; 9, living-chamber; 10, partitions between chambers; 11, siphuncle. 



group. It occurs in the East Indian seas, and, while the 

 shells are very common, the animal is very rare in museums. 

 In geological times allied forms were very abundant, and 

 are known as Ammonites (with tightly coiled shells), and 

 Orthoceratites (with straight shells), etc. 



ORDER II. DIBRANCHIATA. 



These have two gills (one pair), and long, sucker-bearing 

 arms. An ink-sac is always present. The order is sub- 

 divided into the OCTOPODA, in which there are eight arms 

 (fig. 47), and the DECAPODA, in which the number is 

 increased to ten by the addition of a pair of longer arms. 

 In the Octopoda there is no internal shell, and the body 



