MOLLUSCA. 



229 



Pond snails (chiefly) Family Limneidae. 



Sharp; right-handed spiral Genus Limnea. 



Sharp; left-handed spiral Genus Physa. 



Flat coil (like watch spring) Genus Planorbis. 



Gills; marine. 



Gills in front of heart ; operculum present, 



Order Prosobranchiata . 



Gills behind heart ; no operculum. Order Opisthobranciata . 

 Foot modified into tentacles about the mouth, 



Class Cephalopoda. 



FIG. 77. Pearly Nautilus. From Nicholson, e, eye; h, hood, a muscular portion 

 of foot which protects the softer parts; s, siphon; se, septa, separating the successive 

 chambers of the shell; sp, siphuncle; t, tentacles divisions of the foot.' 



Questions on the Text. How does the shell compare with those 

 of gasteropods? Why are the tentacles (arms) thought to be 

 homologous with the foot of the snail? What is the siphuncle? 

 What is the character of the eye in Nautilus? 



Four gills; many-chambered shell Tetrabranchiata. 



Example: Chambered-Nautilus (Fig. 77). 

 Two gills. 



Ten tentacles Order Decapoda. 



Examples : Cuttle-fish ; squid. 



Eight tentacles Order Octapoda. 



Examples: Octopus (devil-fish, Fig. 78) ; Paper Nautilus. 



