MOLLUSCA. 259 



suckers. Shells external and chambered (and in Nautilus, the only living 

 genus, coiled). Two pairs of auricles; two pairs of gills; two pairs of 

 nephridia. 



This group is important' for its extinct rather than for its living repre- 

 sentatives. The pearly or chambered nautilus (Fig. 121) found in the 

 Pacific and Indian Oceans, is the only important living species. The 

 Nautilus appears to be the only remaining descendant of the once numer- 

 ous family of Ammonites and more remotely still of the Orthoceratites, the 

 rulers of the Palaeozoic seas (see Geology). 



Subclass II. Dibranchiata. Cephalopods in which a circlet of 8 to 10 

 arms surround the mouth. These bear sucking discs. Shell internal and 

 rudimentary^or absent. One pair of gills, one pair of nephridia, and one 

 pair of auricles. An ink gland is present. 



Order I, Decapoda, embraces the cuttle-fish and squid. 



Order 2, Octopoda, embraces the devil-fishes (Fig. 122) and the paper 

 nautilus (Fig. 123). 



296. Supplementary Studies for Library, Laboratory, 

 and Field. 



1. Compare the clam, snail, and squid with regard to the 

 following particulars, putting the results in a tabular form : 



(a) Degree of development of the head. 



(&) Shell, development and method of using, in each. 



(c) Mantle; extent, form and modifications: mantle cavity. 



(d) Foot; parts, differentiation, and uses. 



(e) Respiration; how accomplished? 



(/) Sense organs; position, character, and degree of de- 

 velopment. 



(g) Locomotion ; N how effected? 

 (h) Protection; special devices. 



2. Can you find any indication among the mollusks of a 

 relation between the degree of development of the sense 

 organs and the activity shown by the animals? Between the 

 external protective structures and activity? 



3. When did the various classes of mollusks make their 

 appearance in the history of the earth? (See geology.) What 

 can you say of their importance in the formation of the sedi- 

 mentary rock? 



4. In what ways may the fresh-water forms have arisen 

 from the original salt-water mollusks ? 



