MOLLUSCA 



263 



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. 124) and the paper nautilus 

 (Fig. 125). 



FIG. 125. 



FIG. 125. The Paper Nautilus (Argonauta argo). From Cooke, after Lacaze-Duthiers. , eye; 

 ra, mouth;/, siphon; sh, shell; /, tentacles. 



Questions on the figure. In what way does the siphon serve in locomotion? 

 In which direction will the animal move by means of the siphon? How does the 

 shell of Argonauta differ from that of Nautilus? 



303. 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. 



(b) 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 develop- 

 ment. 



(g) Locomotion ; how effected ? 

 (ti) Protection; special devices. 



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

 relation between the degree of development of the sense organs 



