148 MOLLUSKS 



Heart with two auricles; two nephridia Haliotis. 



Heart with a single auricle and a single nephridium. 

 V. Class Cephalopoda.^ — Visceral hump developed; bilaterally sym- 

 metrical ; mantle a circular fold ; foot (propodium and mesopo- 

 dium) forming armlike structures provided with suckers and sur- 

 rounding the mouth. 



1. Order Teteabranchia. — With four ctenidia and with external 

 chambered shell Nautilus. 



2. Order Dibeanchia. — With two ctenidia ; shell if external not 

 chambered, usually internal. 



With eight arms to foot (Octopoda) Octopus (Devil Fish). 



With ten arms to foot (Decapoda) Loligo (Squid). 



