KEY TO CLASSES AND ORDERS OF ANIMALS 17 



2(1) Fresh- water animals with a horseshoe-shaped or oval 

 lophophore; projection present over the mouth; 

 statoblasts usually present. (Pratt, 262; W. and W., 

 952.) Order Phylactolaemata 



Phylum ANNELIDA 



1(2) Bristles (setae) absent; suckers present; leeches. 



Class Hirudinea, p. 17 



2(1) Setae present; suckers absent (except in one parasitic 

 family). Class Chaetopoda, p. 17 



Class CHAETOPODA 



1(4) Segmentation distinct as shown by setae or external 

 rings or by internal arrangement; usually by all 

 three. 2 



2(3) Setae borne on parapodia; usually with special append- 

 ages in the head region; mostly marine. (Pratt, 282.) 



Order Polychaeta 



3(2) No parapodia present; special appendages usually 

 lacking from the head region; mostly fresh water and 

 terrestrial. (Pratt, 303; W. and W., 638.) 



« Order Oligochaeta 



4(1) Segmentation indistinct or wanting in adult, marine. 

 (Pratt, 314.) Order Echiurida 



Class HIRUDINEA 



1(2) Mouth a small pore in oral sucker, from which a 

 muscular proboscis may be protruded; somites rarely 

 consisting of five rings each; blood colorless. (Pratt, 

 317; W. and W., 651.)^ Order Rhynchobdellida 



2(1) Mouth large, occupying the entire cavity of the 

 sucker; no proboscis; usually three teeth present; 

 red blood; somites usually with five rings each. 

 (Pratt, 319; W. and W., 656.) Order Gnathobdellida 



