KEY TO CLASSES AND ORDERS OF ANIMALS 19 



6(1) Distinct head present; shell, if present, usually spirally 

 coiled. 7 



7(8) Head with one or two pairs of tentacles, eyes small if 

 present and either sessile or stalked. Shell, if present, 

 spirally coiled or conic. Snails, slugs, etc. 



Class Gastropoda, p. 20 



8(7) Head provided with eight or ten long arms bearing 

 cuplike suckers. Shell usually apparently absent or 

 if present coiled in flat coil and divided into chambers. 

 Marine forms; squid, octopus, chambered nautilus. 

 (Pratt, 605.) Class Cephalopoda 



Class PELECYPODA 



1(2) Gills absent, replaced by a horizontal septum extending 

 from each side of the body to the mantle fold; marine. 

 (Pratt, 601.) Order Septibranchiata 



2(1) Gills present, marine or fresh water. 3 



3(4) Gills consisting of a single pair of plumelike organs, 

 each with two rows of flattened filaments. Two 

 adductor muscles. Foot with a creeping surface; 

 marine. (Pratt, 567.) Order Protobranchiata 



4(3) With two pairs of flattened gills composed of numerous 

 elongated parallel filaments, which extend ventrally 

 from the base of each gill to its free margin, then 

 dorsally on the opposite side. Thus each filament is 

 more or less V-shaped and each gill is composed of two 

 lamellae, or plates, formed respectively by the descend- 

 ing and ascending limbs of the filaments. 5 



5(6, 7) Marine. Adjacent filaments not united; sometimes 

 held together by interlocking cilia. Usually two 

 adductor muscles. Foot usually possessing a well- 

 developed byssus for attachment. (Pratt, 570.) 



Order Filibranchiata 



